sequencer: rectify empty hint in call of require_clean_work_tree()

sequencer: rectify empty hint in call of require_clean_work_tree()

The canonical way to represent "no error hint" is making it NULL, which
shortcuts the error() call altogether. This fixes the output by removing
the line which said just "error:", which would appear when the worktree
is dirtied while editing the initial rebase todo file. This was
introduced by 97e1873 (rebase -i: rewrite complete_action() in C,
2018-08-28), which did a somewhat inaccurate conversion from shell.

To avoid that such bugs re-appear, test for the condition in
require_clean_work_tree().

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>