It's been a while, let's try again.

Changes:
- Use our own zlib helpers instead of the gz* functions of zlib,
- ... which allows us to set the OS_CODE header consistently.
- Pseudo-command "git archive gzip" to select the internal
  implementation in config.
- Use a function pointer to plug in the internal gzip.
- Tests.
- Discuss performance in commit message.

  archive: rename archiver data field to filter_command
  archive-tar: factor out write_block()
  archive-tar: add internal gzip implementation
  archive-tar: use OS_CODE 3 (Unix) for internal gzip
  archive-tar: use internal gzip by default

 Documentation/git-archive.txt |  3 +-
 archive-tar.c                 | 79 ++++++++++++++++++++++++++++++-----
 archive.h                     |  2 +-
 t/t5000-tar-tree.sh           | 28 ++++++++++---
 4 files changed, 93 insertions(+), 19 deletions(-)

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2.36.1