rebase -r: do not (re-)generate root commits with `--root` *and* `--onto`

When rebasing a complete commit history onto a given commit, it is
pretty obvious that the root commits should be rebased on top of said
given commit.

To test this, let's kill two birds with one stone and add a test case to
t3427-rebase-subtree.sh that not only demonstrates that this works, but
also that `git rebase -r` works with merge strategies now.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -62,7 +62,7 @@ struct rebase_options {
 	const char *onto_name;
 	const char *revisions;
 	const char *switch_to;
-	int root;
+	int root, root_with_onto;
 	struct object_id *squash_onto;
 	struct commit *restrict_revision;
 	int dont_finish_rebase;
@@ -374,6 +374,7 @@ static int run_rebase_interactive(struct rebase_options *opts,
 	flags |= abbreviate_commands ? TODO_LIST_ABBREVIATE_CMDS : 0;
 	flags |= opts->rebase_merges ? TODO_LIST_REBASE_MERGES : 0;
 	flags |= opts->rebase_cousins > 0 ? TODO_LIST_REBASE_COUSINS : 0;
+	flags |= opts->root_with_onto ? TODO_LIST_ROOT_WITH_ONTO : 0;
 	flags |= command == ACTION_SHORTEN_OIDS ? TODO_LIST_SHORTEN_IDS : 0;
 
 	switch (command) {
@@ -1841,7 +1842,9 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			options.squash_onto = &squash_onto;
 			options.onto_name = squash_onto_name =
 				xstrdup(oid_to_hex(&squash_onto));
-		}
+		} else
+			options.root_with_onto = 1;
+
 		options.upstream_name = NULL;
 		options.upstream = NULL;
 		if (argc > 1)

sequencer.c

@@ -4440,6 +4440,7 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 {
 	int keep_empty = flags & TODO_LIST_KEEP_EMPTY;
 	int rebase_cousins = flags & TODO_LIST_REBASE_COUSINS;
+	int root_with_onto = flags & TODO_LIST_ROOT_WITH_ONTO;
 	struct strbuf buf = STRBUF_INIT, oneline = STRBUF_INIT;
 	struct strbuf label = STRBUF_INIT;
 	struct commit_list *commits = NULL, **tail = &commits, *iter;
@@ -4606,7 +4607,8 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 
 		if (!commit)
 			strbuf_addf(out, "%s %s\n", cmd_reset,
-				    rebase_cousins ? "onto" : "[new root]");
+				    rebase_cousins || root_with_onto ?
+				    "onto" : "[new root]");
 		else {
 			const char *to = NULL;
 

sequencer.h

@@ -142,6 +142,12 @@ int sequencer_remove_state(struct replay_opts *opts);
  */
 #define TODO_LIST_REBASE_COUSINS (1U << 4)
 #define TODO_LIST_APPEND_TODO_HELP (1U << 5)
+/*
+ * When generating a script that rebases merges with `--root` *and* with
+ * `--onto`, we do not want to re-generate the root commits.
+ */
+#define TODO_LIST_ROOT_WITH_ONTO (1U << 6)
+
 
 int sequencer_make_script(struct repository *r, struct strbuf *out, int argc,
 			  const char **argv, unsigned flags);

t/t3427-rebase-subtree.sh

@@ -93,4 +93,15 @@ test_expect_success 'Rebase -Xsubtree --keep-empty --onto commit' '
 	verbose test "$(commit_message HEAD)" = "Empty commit"
 '
 
+test_expect_success 'Rebase -Xsubtree --keep-empty --rebase-merges --onto commit' '
+	reset_rebase &&
+	git checkout -b rebase-merges-onto to-rebase &&
+	test_must_fail git rebase -Xsubtree=files_subtree --keep-empty --rebase-merges --onto files-master --root &&
+	: first pick results in no changes &&
+	git rebase --continue &&
+	verbose test "$(commit_message HEAD~2)" = "master4" &&
+	verbose test "$(commit_message HEAD~)" = "files_subtree/master5" &&
+	verbose test "$(commit_message HEAD)" = "Empty commit"
+'
+
 test_done