revisions: initialize revs->grep_filter using grep_init()

revisions: initialize revs->grep_filter using grep_init()

Instead of using the hand-rolled initialization sequence,
use grep_init() to populate the necessary bits.  This opens
the door to allow the calling commands to optionally read
grep.* configuration variables via git_config() if they
want to.

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