rebase -m: do not trigger pre-commit verification

rebase -m: do not trigger pre-commit verification

When rebasing changes that contain issues that the pre-commit hook flags
as problematic, the rebase cannot be continued.  However, rebase is about
transplanting commits that are already made with as little distortion as
possible, and pre-commit check should not interfere.

Earlier, c5b09fe (Avoid update hook during git-rebase --interactive,
2007-12-19) fixed "rebase -i", but "rebase -m" shared the same issue.

Signed-off-by: Junio C Hamano <gitster@pobox.com>