sequencer (rebase -i): allow rescheduling commands

The interactive rebase has the very special magic that a cherry-pick
that exits with a status different from 0 and 1 signifies a failure to
even record that a cherry-pick was started.

This can happen e.g. when a fast-forward fails because it would
overwrite untracked files.

In that case, we must reschedule the command that we thought we already
had at least started successfully.

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

sequencer.c

@@ -1915,6 +1915,12 @@ static int pick_commits(struct todo_list *todo_list, struct replay_opts *opts)
 					1);
 			res = do_pick_commit(item->command, item->commit,
 					opts, is_final_fixup(todo_list));
+			if (is_rebase_i(opts) && res < 0) {
+				/* Reschedule */
+				todo_list->current--;
+				if (save_todo(todo_list, opts))
+					return -1;
+			}
 			if (item->command == TODO_EDIT) {
 				struct commit *commit = item->commit;
 				if (!res)