perf: amend the grep tests to test grep.threads

perf: amend the grep tests to test grep.threads

Ever since 5b594f457a ("Threaded grep", 2010-01-25) the number of
threads git-grep uses under PTHREADS has been hardcoded to 8, but
there's no performance test to check whether this is an optimal
setting.

Amend the existing tests for the grep engines to support a mode where
this can be tested, e.g.:

    GIT_PERF_GREP_THREADS='1 8 16' GIT_PERF_LARGE_REPO=~/g/linux ./run p782*

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>