Fix prepare-commit-msg hook and replace in-place sed

The patterns to the case statement could never be matched, so the hook
was a noop. This patch also replaces the non-portable use of in-place sed.

Signed-off-by: Marcel Koeppen <git-dev@marzelpan.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>