hashcmp: use memcmp instead of open-coded loop

In 1a812f3a70 (hashcmp(): inline memcmp() by hand to
optimize, 2011-04-28), it was reported that an open-coded
loop outperformed memcmp() for comparing sha1s.

Discussion[1] a few years later in 2013 showed that this
depends on your libc's version of memcmp(). In particular,
glibc 2.13 optimized their memcmp around 2011. Here are
current timings with glibc 2.24 (best-of-five, on
linux.git):

  [before this patch, open-coded]
  $ time git rev-list --objects --all
  real	0m35.357s
  user	0m35.016s
  sys	0m0.340s

  [after this patch, memcmp]
  real	0m32.930s
  user	0m32.630s
  sys	0m0.300s

Now that we've had 6 years for that version of glibc to
make its way onto people's machines, it's worth revisiting
our benchmarks and switching to memcmp().

It may be that there are other non-glibc systems where
memcmp() isn't as well optimized. But since our single data
point in favor of open-coding was on a now-ancient glibc, we
should probably assume the system memcmp is good unless
proven otherwise. We may end up with a SLOW_MEMCMP Makefile
knob, but we can hold off on that until we actually find
such a system in practice.

[1] https://public-inbox.org/git/20130318073229.GA5551@sigill.intra.peff.net/

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

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