Merge branch 'rs/archive-with-internal-gzip'

Teach "git archive" to (optionally and then by default) avoid
spawning an external "gzip" process when creating ".tar.gz" (and
".tgz") archives.

* rs/archive-with-internal-gzip:
  archive-tar: use internal gzip by default
  archive-tar: use OS_CODE 3 (Unix) for internal gzip
  archive-tar: add internal gzip implementation
  archive-tar: factor out write_block()
  archive: rename archiver data field to filter_command
  archive: update format documentation