Merge branch 'jk/pack-header-parse-alignment-fix'

It was possible for "git unpack-objects" and "git index-pack" to
make an unaligned access, which has been corrected.

* jk/pack-header-parse-alignment-fix:
  index-pack, unpack-objects: use skip_prefix to avoid magic number
  index-pack, unpack-objects: use get_be32() for reading pack header
  parse_pack_header_option(): avoid unaligned memory writes
  packfile: factor out --pack_header argument parsing
  bswap.h: squelch potential sparse -Wcast-truncate warnings

diff --cc builtin/index-pack.c
Simple merge

diff --cc builtin/unpack-objects.c
Simple merge

diff --cc pack.h
Simple merge

diff --cc packfile.c
Simple merge

diff --cc packfile.h
Simple merge