grep: allow to use textconv filters

Recently and not so recently, we made sure that log/grep type operations
use textconv filters when a userfacing diff would do the same:

ef90ab66 (pickaxe: use textconv for -S counting, 2012-10-28)
b1c2f57d (diff_grep: use textconv buffers for add/deleted files, 2012-10-28)
0508fe53 (combine-diff: respect textconv attributes, 2011-05-23)

"git grep" currently does not use textconv filters at all, that is
neither for displaying the match and context nor for the actual grepping,
even when requested by --textconv.

Introduce an option "--textconv" which makes git grep use any configured
textconv filters for grepping and output purposes. It is off by default.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>