grep: skip pthreads overhead when using one thread

Skip the administrative overhead of using pthreads when only using one thread. Instead take the non-threaded path which would be taken under NO_PTHREADS. The threading support was initially added in commit 5b594f457a ("Threaded grep", 2010-01-25) with a hardcoded compile-time number of 8 threads. Later the number of threads was made configurable in commit 89f09dd34e ("grep: add --threads=<num> option and grep.threads configuration", 2015-12-15). That change did not add any special handling for --threads=1. Now we take a slightly faster path by skipping thread handling entirely when 1 thread is requested. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>