Sha 256 support

[Byron]

Even though the foundation is set, it needs another push to actually make it work with different kinds of hashes.

Tasks

• remove hash-type specific methods from git-hash and replace them with parametric usage of git_hash::Kind
• all code assuming hashes of len 20 should receive this value as parameter instead. This is what git does for the old index and pack file formats.
• a way to pass --object-hash information to the gix CLI
• remove SHA1 mention from git-features feature toggles
• parameterize hash len when decoding non-blob objects (see this for an example)
• understand and implement pack idx V3. - see if git actually implements this, and maybe decide that gitoxide won't handle the transition period, is either one has or another.
• add new Sha256 enum variant, consider putting it behind a feature flag, and add a hasher for it as well.
• general tests for reading refs and objects of different len
• tests for writing and reading objects of different len, maybe even write a conversion program which transforms an entire repo and double-checks with git-fsck
• when cloning, check the uninmplemented!() invocation to configure the repo for expecting a different hash

Implementation ideas

• make sure once Sha256 is added as ObjectId variant, that it's behind a feature toggle to allow builds that opt-out of SHA256 support to not unnecessarily use more memory than needed. Maybe there are alternatives to this, too.

• One way to do that with approximately zero overhead would be to such functions generic on the object ID, using a trait that has a method to get the type. Then object IDs with a known type return a constant from that method, and object IDs with a runtime dispatched type return the value of that enum.

  • @joshtriplett - taken verbatim as I'd barely be able to improve on it when paraphrasing. In short, have a trait for oid or allow efficient conversions to oid (it's just a slice, so that should work for specifically sized types as well especially if these were provided by git-hash.

Notes

• find ways to use the existing highly-parallel pack traversal (along with integration of loose-objects) to build an inverse-ref table to quickly traverse objects bottom-up to change the hash used along with all references, while being fast. This ties into being able to build new packs quickly, ideally even with delta-compression (the latter then has to be re-created as most objects actually change) - re-using deltas for blobs is the only way.
  • The existing traversal can mutate data in the tree, which is enough to decode the object and keep direct references for later.

[shea256]

+1 to this feature. Excited to see this implemented.

[msrd0]

+1. Given that git no longer considers sha256 support experimental and forges such as forgejo added support, this has became more relevant for new git repositories.