rebase --rebase-merges: avoid "empty merges"

The `git merge` command does not allow merging commits that are already
reachable from HEAD: `git merge HEAD^`, for example, will report that we
are already up to date and not change a thing.

In an interactive rebase, such a merge could occur previously, e.g. when
competing (or slightly modified) versions of a patch series were applied
upstream, and the user had to `git rebase --skip` all of the local
commits, and the topic branch becomes "empty" as a consequence.

Let's teach the todo command `merge` to behave the same as `git merge`.

Seeing as it requires some low-level trickery to create such merges with
Git's commands in the first place, we do not even have to bother to
introduce an option to force `merge` to create such merge commits.

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

sequencer.c

@@ -2810,6 +2810,13 @@ static int do_merge(struct commit *commit, const char *arg, int arg_len,
 	write_message("no-ff", 5, git_path_merge_mode(), 0);
 
 	bases = get_merge_bases(head_commit, merge_commit);
+	if (bases && !oidcmp(&merge_commit->object.oid,
+			     &bases->item->object.oid)) {
+		ret = 0;
+		/* skip merging an ancestor of HEAD */
+		goto leave_merge;
+	}
+
 	for (j = bases; j; j = j->next)
 		commit_list_insert(j->item, &reversed);
 	free_commit_list(bases);

t/t3430-rebase-merges.sh

@@ -215,4 +215,12 @@ test_expect_success 'post-rewrite hook and fixups work for merges' '
 	test_cmp expect actual
 '
 
+test_expect_success 'refuse to merge ancestors of HEAD' '
+	echo "merge HEAD^" >script-from-scratch &&
+	test_config -C wt sequence.editor \""$PWD"/replace-editor.sh\" &&
+	before="$(git rev-parse HEAD)" &&
+	git rebase -i HEAD &&
+	test_cmp_rev HEAD $before
+'
+
 test_done