sequencer (rebase -i): the todo can be empty when continuing

When the last command of an interactive rebase fails, the user needs to
resolve the problem and then continue the interactive rebase. Naturally,
the todo script is empty by then. So let's not complain about that!

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

sequencer.c

@@ -1153,7 +1153,8 @@ static int parse_insn_buffer(char *buf, struct todo_list *todo_list,
 			fixup_okay = 1;
 		p = *eol ? eol + 1 : eol;
 	}
-	if (!todo_list->nr)
+	if (!todo_list->nr && (opts->action != REPLAY_INTERACTIVE_REBASE ||
+			!file_exists(git_path_rebase_done())))
 		return error(_("No commits parsed."));
 	return 0;
 }