rebase: add a config option for --rebase-merges

The purpose of the new option is to accommodate users who would like
--rebase-merges to be on by default and to facilitate turning on
--rebase-merges by default without configuration in a future version of
Git.

Name the new option rebase.rebaseMerges, even though it is a little
redundant, for consistency with the name of the command line option and
to be clear when scrolling through values in the [rebase] section of
.gitconfig.

Support setting rebase.rebaseMerges to the nonspecific value "true" for
users who don't need to or don't want to learn about the difference
between rebase-cousins and no-rebase-cousins.

Make --rebase-merges without an argument on the command line override
any value of rebase.rebaseMerges in the configuration, for consistency
with other command line flags with optional arguments that have an
associated config option.

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/rebase.txt

@@ -67,3 +67,13 @@ rebase.rescheduleFailedExec::
 
 rebase.forkPoint::
 	If set to false set `--no-fork-point` option by default.
+
+rebase.rebaseMerges::
+	Whether and how to set the `--rebase-merges` option by default. Can
+	be `rebase-cousins`, `no-rebase-cousins`, or a boolean. Setting to
+	true or to `no-rebase-cousins` is equivalent to
+	`--rebase-merges=no-rebase-cousins`, setting to `rebase-cousins` is
+	equivalent to `--rebase-merges=rebase-cousins`, and setting to false is
+	equivalent to `--no-rebase-merges`. Passing `--rebase-merges` on the
+	command line, with or without an argument, overrides any
+	`rebase.rebaseMerges` configuration.

Documentation/git-rebase.txt

@@ -537,7 +537,8 @@ See also INCOMPATIBLE OPTIONS below.
 	by recreating the merge commits. Any resolved merge conflicts or
 	manual amendments in these merge commits will have to be
 	resolved/re-applied manually. `--no-rebase-merges` can be used to
-	countermand a previous `--rebase-merges`.
+	countermand both the `rebase.rebaseMerges` config option and a previous
+	`--rebase-merges`.
 +
 When rebasing merges, there are two modes: `rebase-cousins` and
 `no-rebase-cousins`. If the mode is not specified, it defaults to

builtin/rebase.c

@@ -124,6 +124,7 @@ struct rebase_options {
 	int fork_point;
 	int update_refs;
 	int config_autosquash;
+	int config_rebase_merges;
 	int config_update_refs;
 };
 
@@ -141,6 +142,8 @@ struct rebase_options {
 		.allow_empty_message = 1,               \
 		.autosquash = -1,                       \
 		.config_autosquash = -1,                \
+		.rebase_merges = -1,                    \
+		.config_rebase_merges = -1,             \
 		.update_refs = -1,                      \
 		.config_update_refs = -1,               \
 	}
@@ -772,6 +775,16 @@ static int run_specific_rebase(struct rebase_options *opts)
 	return status ? -1 : 0;
 }
 
+static void parse_rebase_merges_value(struct rebase_options *options, const char *value)
+{
+	if (!strcmp("no-rebase-cousins", value))
+		options->rebase_cousins = 0;
+	else if (!strcmp("rebase-cousins", value))
+		options->rebase_cousins = 1;
+	else
+		die(_("Unknown rebase-merges mode: %s"), value);
+}
+
 static int rebase_config(const char *var, const char *value, void *data)
 {
 	struct rebase_options *opts = data;
@@ -801,6 +814,17 @@ static int rebase_config(const char *var, const char *value, void *data)
 		return 0;
 	}
 
+	if (!strcmp(var, "rebase.rebasemerges")) {
+		opts->config_rebase_merges = git_parse_maybe_bool(value);
+		if (opts->config_rebase_merges < 0) {
+			opts->config_rebase_merges = 1;
+			parse_rebase_merges_value(opts, value);
+		} else {
+			opts->rebase_cousins = 0;
+		}
+		return 0;
+	}
+
 	if (!strcmp(var, "rebase.updaterefs")) {
 		opts->config_update_refs = git_config_bool(var, value);
 		return 0;
@@ -981,6 +1005,28 @@ static int parse_opt_empty(const struct option *opt, const char *arg, int unset)
 	return 0;
 }
 
+static int parse_opt_rebase_merges(const struct option *opt, const char *arg, int unset)
+{
+	struct rebase_options *options = opt->value;
+
+	options->rebase_merges = !unset;
+	options->rebase_cousins = 0;
+
+	if (arg) {
+		if (!*arg) {
+			warning(_("--rebase-merges with an empty string "
+				  "argument is deprecated and will stop "
+				  "working in a future version of Git. Use "
+				  "--rebase-merges without an argument "
+				  "instead, which does the same thing."));
+			return 0;
+		}
+		parse_rebase_merges_value(options, arg);
+	}
+
+	return 0;
+}
+
 static void NORETURN error_on_missing_default_upstream(void)
 {
 	struct branch *current_branch = branch_get(NULL);
@@ -1036,7 +1082,6 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	struct object_id branch_base;
 	int ignore_whitespace = 0;
 	const char *gpg_sign = NULL;
-	const char *rebase_merges = NULL;
 	struct string_list strategy_options = STRING_LIST_INIT_NODUP;
 	struct object_id squash_onto;
 	char *squash_onto_name = NULL;
@@ -1138,10 +1183,9 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			   &options.allow_empty_message,
 			   N_("allow rebasing commits with empty messages"),
 			   PARSE_OPT_HIDDEN),
-		{OPTION_STRING, 'r', "rebase-merges", &rebase_merges,
-			N_("mode"),
+		OPT_CALLBACK_F('r', "rebase-merges", &options, N_("mode"),
 			N_("try to rebase merges instead of skipping them"),
-			PARSE_OPT_OPTARG, NULL, (intptr_t)"no-rebase-cousins"},
+			PARSE_OPT_OPTARG, parse_opt_rebase_merges),
 		OPT_BOOL(0, "fork-point", &options.fork_point,
 			 N_("use 'merge-base --fork-point' to refine upstream")),
 		OPT_STRING('s', "strategy", &options.strategy,
@@ -1437,21 +1481,6 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	if (options.exec.nr)
 		imply_merge(&options, "--exec");
 
-	if (rebase_merges) {
-		if (!*rebase_merges)
-			warning(_("--rebase-merges with an empty string "
-				  "argument is deprecated and will stop "
-				  "working in a future version of Git. Use "
-				  "--rebase-merges without an argument "
-				  "instead, which does the same thing."));
-		else if (!strcmp("rebase-cousins", rebase_merges))
-			options.rebase_cousins = 1;
-		else if (strcmp("no-rebase-cousins", rebase_merges))
-			die(_("Unknown mode: %s"), rebase_merges);
-		options.rebase_merges = 1;
-		imply_merge(&options, "--rebase-merges");
-	}
-
 	if (options.type == REBASE_APPLY) {
 		if (ignore_whitespace)
 			strvec_push(&options.git_am_opts,
@@ -1519,6 +1548,8 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 					  "cannot be used together"));
 			else if (options.autosquash == -1 && options.config_autosquash == 1)
 				die(_("apply options are incompatible with rebase.autoSquash.  Consider adding --no-autosquash"));
+			else if (options.rebase_merges == -1 && options.config_rebase_merges == 1)
+				die(_("apply options are incompatible with rebase.rebaseMerges.  Consider adding --no-rebase-merges"));
 			else if (options.update_refs == -1 && options.config_update_refs == 1)
 				die(_("apply options are incompatible with rebase.updateRefs.  Consider adding --no-update-refs"));
 			else
@@ -1531,6 +1562,11 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	options.update_refs = (options.update_refs >= 0) ? options.update_refs :
 			     ((options.config_update_refs >= 0) ? options.config_update_refs : 0);
 
+	if (options.rebase_merges == 1)
+		imply_merge(&options, "--rebase-merges");
+	options.rebase_merges = (options.rebase_merges >= 0) ? options.rebase_merges :
+				((options.config_rebase_merges >= 0) ? options.config_rebase_merges : 0);
+
 	if (options.autosquash == 1)
 		imply_merge(&options, "--autosquash");
 	options.autosquash = (options.autosquash >= 0) ? options.autosquash :

t/t3422-rebase-incompatible-options.sh

@@ -85,6 +85,11 @@ test_rebase_am_only () {
 		test_must_fail git rebase $opt --reapply-cherry-picks A
 	"
 
+	test_expect_success "$opt incompatible with --rebase-merges" "
+		git checkout B^0 &&
+		test_must_fail git rebase $opt --rebase-merges A
+	"
+
 	test_expect_success "$opt incompatible with --update-refs" "
 		git checkout B^0 &&
 		test_must_fail git rebase $opt --update-refs A
@@ -101,6 +106,12 @@ test_rebase_am_only () {
 		grep -e --no-autosquash err
 	"
 
+	test_expect_success "$opt incompatible with rebase.rebaseMerges" "
+		git checkout B^0 &&
+		test_must_fail git -c rebase.rebaseMerges=true rebase $opt A 2>err &&
+		grep -e --no-rebase-merges err
+	"
+
 	test_expect_success "$opt incompatible with rebase.updateRefs" "
 		git checkout B^0 &&
 		test_must_fail git -c rebase.updateRefs=true rebase $opt A 2>err &&
@@ -113,6 +124,12 @@ test_rebase_am_only () {
 		git -c rebase.autosquash=true rebase --no-autosquash $opt A
 	"
 
+	test_expect_success "$opt okay with overridden rebase.rebaseMerges" "
+		test_when_finished \"git reset --hard B^0\" &&
+		git checkout B^0 &&
+		git -c rebase.rebaseMerges=true rebase --no-rebase-merges $opt A
+	"
+
 	test_expect_success "$opt okay with overridden rebase.updateRefs" "
 		test_when_finished \"git reset --hard B^0\" &&
 		git checkout B^0 &&

t/t3430-rebase-merges.sh

@@ -278,6 +278,40 @@ test_expect_success 'do not rebase cousins unless asked for' '
 	EOF
 '
 
+test_expect_success 'rebase.rebaseMerges=rebase-cousins is equivalent to --rebase-merges=rebase-cousins' '
+	test_config rebase.rebaseMerges rebase-cousins &&
+	git checkout -b config-rebase-cousins main &&
+	git rebase HEAD^ &&
+	test_cmp_graph HEAD^.. <<-\EOF
+	*   Merge the topic branch '\''onebranch'\''
+	|\
+	| * D
+	| * G
+	|/
+	o H
+	EOF
+'
+
+test_expect_success '--no-rebase-merges overrides rebase.rebaseMerges=no-rebase-cousins' '
+	test_config rebase.rebaseMerges no-rebase-cousins &&
+	git checkout -b override-config-no-rebase-cousins E &&
+	git rebase --no-rebase-merges C &&
+	test_cmp_graph C.. <<-\EOF
+	* B
+	* D
+	o C
+	EOF
+'
+
+test_expect_success '--rebase-merges overrides rebase.rebaseMerges=rebase-cousins' '
+	test_config rebase.rebaseMerges rebase-cousins &&
+	git checkout -b override-config-rebase-cousins E &&
+	before="$(git rev-parse --verify HEAD)" &&
+	test_tick &&
+	git rebase --rebase-merges C &&
+	test_cmp_rev HEAD $before
+'
+
 test_expect_success 'refs/rewritten/* is worktree-local' '
 	git worktree add wt &&
 	cat >wt/script-from-scratch <<-\EOF &&