cherry-pick: allow to pick to unborn branches

cherry-pick allows to pick single commits to an empty HEAD, but not
multiple commits.

Allow the multiple commit case, too.

Reported-by: Fabrizio Cucci <fabrizio.cucci@gmail.com>
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -919,6 +919,10 @@ static int sequencer_rollback(struct replay_opts *opts)
 			filename);
 		goto fail;
 	}
+	if (is_null_sha1(sha1)) {
+		error(_("cannot abort from a branch yet to be born"));
+		goto fail;
+	}
 	if (reset_for_rollback(sha1))
 		goto fail;
 	remove_sequencer_state();
@@ -1118,11 +1122,8 @@ int sequencer_pick_revisions(struct replay_opts *opts)
 	walk_revs_populate_todo(&todo_list, opts);
 	if (create_seq_dir() < 0)
 		return -1;
-	if (get_sha1("HEAD", sha1)) {
-		if (opts->action == REPLAY_REVERT)
-			return error(_("Can't revert as initial commit"));
-		return error(_("Can't cherry-pick into empty head"));
-	}
+	if (get_sha1("HEAD", sha1) && (opts->action == REPLAY_REVERT))
+		return error(_("Can't revert as initial commit"));
 	save_head(sha1_to_hex(sha1));
 	save_opts(opts);
 	return pick_commits(todo_list, opts);