Threaded grep

Make git grep use threads when it is available.

The results below are best of five runs in the Linux repository (on a
box with two cores).

With the patch:

git grep qwerty
1.58user 0.55system 0:01.16elapsed 183%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+800outputs (0major+5774minor)pagefaults 0swaps

Without:

git grep qwerty
1.59user 0.43system 0:02.02elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+800outputs (0major+3716minor)pagefaults 0swaps

And with a pattern with quite a few matches:

With the patch:

$ /usr/bin/time git grep void
5.61user 0.56system 0:03.44elapsed 179%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+800outputs (0major+5587minor)pagefaults 0swaps

Without:

$ /usr/bin/time git grep void
5.36user 0.51system 0:05.87elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+800outputs (0major+3693minor)pagefaults 0swaps

In either case we gain about 40% by the threading.

Signed-off-by: Fredrik Kuivinen <frekui@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>