git log: Unify header_filter and message_filter into one.

git log: Unify header_filter and message_filter into one.

Now we can tell the built-in grep to grep only in head or in
body, use that to update --author, --committer, and --grep.

Unfortunately, to make --and, --not and other grep boolean
expressions useful, as in:

	# Things written by Junio committed and by Linus and log
	# does not talk about diff.

	git log --author=Junio --and --committer=Linus \
		--grep-not --grep=diff

we will need to do another round of built-in grep core
enhancement, because grep boolean expressions are designed to
work on one line at a time.

Signed-off-by: Junio C Hamano <junkio@cox.net>