format-patch: introduce format.useAutoBase configuration

This allows to record the base commit automatically, it is equivalent
to set --base=auto in cmdline.

The format.useAutoBase has lower priority than command line option,
so if user set format.useAutoBase and pass the command line option in
the meantime, base_commit will be the one passed to command line
option.

Signed-off-by: Xiaolong Ye <xiaolong.ye@intel.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1258,6 +1258,10 @@ format.outputDirectory::
 	Set a custom directory to store the resulting files instead of the
 	current working directory.
 
+format.useAutoBase::
+	A boolean value which lets you enable the `--base=auto` option of
+	format-patch by default.
+
 filter.<driver>.clean::
 	The command which is used to convert the content of a worktree
 	file to a blob upon checkin.  See linkgit:gitattributes[5] for

builtin/log.c

@@ -696,6 +696,7 @@ static void add_header(const char *value)
 #define THREAD_DEEP 2
 static int thread;
 static int do_signoff;
+static int base_auto;
 static const char *signature = git_version_string;
 static const char *signature_file;
 static int config_cover_letter;
@@ -780,6 +781,10 @@ static int git_format_config(const char *var, const char *value, void *cb)
 	}
 	if (!strcmp(var, "format.outputdirectory"))
 		return git_config_string(&config_output_directory, var, value);
+	if (!strcmp(var, "format.useautobase")) {
+		base_auto = git_config_bool(var, value);
+		return 0;
+	}
 
 	return git_log_config(var, value, cb);
 }
@@ -1199,7 +1204,11 @@ static struct commit *get_base_commit(const char *base_commit,
 	struct commit **rev;
 	int i = 0, rev_nr = 0;
 
-	if (!strcmp(base_commit, "auto")) {
+	if (base_commit && strcmp(base_commit, "auto")) {
+		base = lookup_commit_reference_by_name(base_commit);
+		if (!base)
+			die(_("Unknown commit %s"), base_commit);
+	} else if ((base_commit && !strcmp(base_commit, "auto")) || base_auto) {
 		struct branch *curr_branch = branch_get(NULL);
 		const char *upstream = branch_get_upstream(curr_branch, NULL);
 		if (upstream) {
@@ -1221,10 +1230,6 @@ static struct commit *get_base_commit(const char *base_commit,
 			      "please use git branch --set-upstream-to to track a remote branch.\n"
 			      "Or you could specify base commit by --base=<base-commit-id> manually."));
 		}
-	} else {
-		base = lookup_commit_reference_by_name(base_commit);
-		if (!base)
-			die(_("Unknown commit %s"), base_commit);
 	}
 
 	ALLOC_ARRAY(rev, total);
@@ -1662,7 +1667,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	}
 
 	memset(&bases, 0, sizeof(bases));
-	if (base_commit) {
+	if (base_commit || base_auto) {
 		struct commit *base = get_base_commit(base_commit, list, nr);
 		reset_revision_walk();
 		prepare_bases(&bases, base, list, nr);

t/t4014-format-patch.sh

@@ -1546,4 +1546,23 @@ test_expect_success 'format-patch errors out when history involves criss-cross'
 	test_must_fail 	git format-patch --base=auto -1
 '
 
+test_expect_success 'format-patch format.useAutoBaseoption' '
+	test_when_finished "git config --unset format.useAutoBase" &&
+	git checkout local &&
+	git config format.useAutoBase true &&
+	git format-patch --stdout -1 >patch &&
+	grep "^base-commit:" patch >actual &&
+	echo "base-commit: $(git rev-parse upstream)" >expected &&
+	test_cmp expected actual
+'
+
+test_expect_success 'format-patch --base overrides format.useAutoBase' '
+	test_when_finished "git config --unset format.useAutoBase" &&
+	git config format.useAutoBase true &&
+	git format-patch --stdout --base=HEAD~1 -1 >patch &&
+	grep "^base-commit:" patch >actual &&
+	echo "base-commit: $(git rev-parse HEAD~1)" >expected &&
+	test_cmp expected actual
+'
+
 test_done