sequencer (rebase -i): leave a patch upon error

When doing an interactive rebase, we want to leave a 'patch' file for
further inspection by the user (even if we never tried to actually apply
that patch, since we're cherry-picking instead).

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

sequencer.c

@@ -1769,7 +1769,9 @@ static int pick_commits(struct todo_list *todo_list, struct replay_opts *opts)
 					intend_to_amend();
 				return error_failed_squash(item->commit, opts,
 					item->arg_len, item->arg);
-			}
+			} else if (res && is_rebase_i(opts))
+				return res | error_with_patch(item->commit,
+					item->arg, item->arg_len, opts, res, 0);
 		} else if (item->command == TODO_EXEC) {
 			char *end_of_arg = (char *)(item->arg + item->arg_len);
 			int saved = *end_of_arg;