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