accept "git grep -- pattern"

accept "git grep -- pattern"

Currently the only way to "quote" a grep pattern that might
begin with a dash is to use "git grep -e pattern". This
works just fine, and is also the way right way to do it on
many traditional grep implemenations.

Some people prefer to use "git grep -- pattern", however, as
"--" is the usual "end of options" marker, and at least GNU
grep and Solaris 10 grep support this. This patch makes that
syntax work.

There is a slight behavior change, in that "git grep -- $X"
used to be interpreted as "grep for -- in $X". However, that
usage is questionable. "--" is usually the end-of-options
marker, so "git grep" was unlike many other greps in
treating it as a literal pattern (e.g., both GNU grep and
Solaris 10 grep will treat "grep --" as missing a pattern).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>