Implement SHA256 support. High level changes: - [x] Repository Format Extension - [x] Packfile - [x] [Support for representing SHA1 and SHA256 objects concurrently](https://github.com/go-git/go-git/pull/1527). - [x] [Support for rev files using SHA1 or SHA256 objects](https://github.com/go-git/go-git/pull/1731). - [x] Loose object index - [x] Refactor go-git-fixtures and expand test coverage for `sha256`. Operations: - [x] [Init/Open a valid SHA-256 repository](https://github.com/go-git/go-git/pull/1667) (Same for [PlainInit/PlainOpen](https://github.com/go-git/go-git/pull/1678)) - [x] Clone - [x] [Open an existing repository](https://github.com/go-git/go-git/pull/1527) - [x] Blame - [ ] Signed Commits - [ ] Signed Tags - [ ] Mergetags - [x] [Submodules](https://github.com/go-git/go-git/pull/1979) - [ ] Shallow clone (unsupported - fail gracefully)
Implement SHA256 support.
High level changes:
sha256.Operations: