Merge branch 'lt/block-sha1'

* lt/block-sha1:
  remove ARM and Mozilla SHA1 implementations
  block-sha1: guard gcc extensions with __GNUC__
  make sure byte swapping is optimal for git
  block-sha1: make the size member first in the context struct

diff --cc Makefile
Simple merge

diff --cc git-compat-util.h
Simple merge