sequencer: drop "warning:" when stopping for edit

Since the conversion from shell to C in 56dc3ab04 (sequencer
(rebase -i): implement the 'edit' command, 2017-01-02),
stopping at an "edit" instruction went from:

  $ git rebase -i
  Stopped at 6ce6b914a... odb_pack_keep(): stop generating keepfile name
  You can amend the commit now, with
    [...more instructions...]

to:

  $ git rebase -i
  warning: stopped at 6ce6b914a... odb_pack_keep(): stop generating keepfile name
  You can amend the commit now, with
    [...more instructions...]

The "warning" implies that it's something unexpected, but
it's not. Let's switch back to the original message.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -1997,7 +1997,8 @@ static int pick_commits(struct todo_list *todo_list, struct replay_opts *opts)
 			if (item->command == TODO_EDIT) {
 				struct commit *commit = item->commit;
 				if (!res)
-					warning(_("stopped at %s... %.*s"),
+					fprintf(stderr,
+						_("Stopped at %s...  %.*s"),
 						short_commit_name(commit),
 						item->arg_len, item->arg);
 				return error_with_patch(commit,