builtin-grep: workaround for non GNU grep.

builtin-grep: workaround for non GNU grep.

Of course, it still ignores the fact that not all grep's support some of
the flags like -F/-L/-A/-C etc, but for those cases, the external grep
itself will happily just say "unrecognized option -F" or similar.

So with this change, "git grep" should handle all the flags the native
grep handles, which is really quite fine. We don't _need_ to expose
anything more, and if you do want our extensions, you can get them with
"--uncached" and an up-to-date index.

No configuration necessary, and we automatically take advantage of any
native grep we have, if possible.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>