support manyfiles

[rggjan]

dulwich doesn't support manyfiles: https://git-scm.com/docs/git-config#Documentation/git-config.txt-featuremanyFiles

manyfiles enables some features which make working with git faster in large repositories.

However, this enables version 4, which then crashes dulwich (see also issue #1061)

Also, due to index.skipHash=true, dulwich crashes when opening the index:

  File ".../lib/python3.10/site-packages/dulwich/pack.py", line 1440, in check_sha
    raise ChecksumMismatch(self.sha1.hexdigest(), sha_to_hex(stored))
dulwich.errors.ChecksumMismatch: Checksum mismatch: Expected 2580840b6abffb2b98771d0a69884a5afa388ec, got b'0000000000000000000000000000000000000000'

[jelmer]

PRs to add support would be very welcome. Happy to talk through designs if that would be useful.