rebase -i: include MERGE_HEAD into files to clean up

Every once in a while, the interactive rebase makes sure that no stale
files are lying around. These days, we need to include MERGE_HEAD into
that set of files, as the `merge` command will generate them.

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

sequencer.c

@@ -3459,6 +3459,7 @@ static int pick_commits(struct todo_list *todo_list, struct replay_opts *opts)
 			unlink(rebase_path_author_script());
 			unlink(rebase_path_stopped_sha());
 			unlink(rebase_path_amend());
+			unlink(git_path_merge_head(the_repository));
 			delete_ref(NULL, "REBASE_HEAD", NULL, REF_NO_DEREF);
 
 			if (item->command == TODO_BREAK)
@@ -3829,6 +3830,7 @@ static int commit_staged_changes(struct replay_opts *opts,
 			   opts, flags))
 		return error(_("could not commit staged changes."));
 	unlink(rebase_path_amend());
+	unlink(git_path_merge_head(the_repository));
 	if (final_fixup) {
 		unlink(rebase_path_fixup_msg());
 		unlink(rebase_path_squash_msg());