chainlint.pl: only start threads if jobs > 1

chainlint.pl: only start threads if jobs > 1

If the system supports threads, chainlint.pl will always spawn worker
threads to do the real work. But when --jobs=1, this is pointless, since
we could just do the work in the main thread. And spawning even a single
thread has a high overhead. For example, on my Linux system, running:

  for i in chainlint/*.test; do
	perl chainlint.pl --jobs=1 $i
  done >/dev/null

takes ~1.7s without this patch, and ~1.1s after. We don't usually spawn
a bunch of individual chainlint.pl processes (instead we feed several
scripts at once, and the parallelism outweighs the setup cost). But it's
something we've considered doing, and since we already have fallback
code for systems without thread support, it's pretty easy to make this
work.

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