This one includes the Mozilla SHA1 implementation sent in by Edgar Toernig.
It's dual-licenced under MPL-1.1 or GPL, so in the context of git, we
obviously use the GPL version.
Side note: the Mozilla SHA1 implementation is about twice as fast as the
default openssl one on my G5, but the default openssl one has optimized
x86 assembly language on x86. So choose wisely.
