commit: add --cleanup=scissors

commit: add --cleanup=scissors

Since 1a72cfd (commit -v: strip diffs and submodule shortlogs from the
commit message - 2013-12-05) we have a less fragile way to cut out
"git status" at the end of a commit message but it's only enabled for
stripping submodule shortlogs.

Add new cleanup option that reuses the same mechanism for the entire
"git status" without accidentally removing lines starting with '#'.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>