sequencer: ensure to release the lock when we could not read the index

A future caller of read_and_refresh_cache() may want to do more than just
print some helpful advice in case of failure.

Suggested by Junio Hamano.

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

sequencer.c

@@ -644,15 +644,19 @@ static int read_and_refresh_cache(struct replay_opts *opts)
 {
 	static struct lock_file index_lock;
 	int index_fd = hold_locked_index(&index_lock, 0);
-	if (read_index_preload(&the_index, NULL) < 0)
+	if (read_index_preload(&the_index, NULL) < 0) {
+		rollback_lock_file(&index_lock);
 		return error(_("git %s: failed to read the index"),
 			action_name(opts));
+	}
 	refresh_index(&the_index, REFRESH_QUIET|REFRESH_UNMERGED, NULL, NULL, NULL);
 	if (the_index.cache_changed && index_fd >= 0) {
-		if (write_locked_index(&the_index, &index_lock, COMMIT_LOCK))
+		if (write_locked_index(&the_index, &index_lock, COMMIT_LOCK)) {
+			rollback_lock_file(&index_lock);
 			return error(_("git %s: failed to refresh the index"),
 				action_name(opts));
 		}
+	}
 	rollback_lock_file(&index_lock);
 	return 0;
 }