Enable unstable-sha256 for git2

[cruessler]

This PR is a follow-up to #2962 and #2967. It enables the unstable-sha256 feature for git2. This change should enable gitui to work with SHA-256 repos since SHA-256 support for gitoxide was enabled via #2967 already.

[cruessler]

@extrawurst I tested this branch locally and was able to open a SHA-256 repository. I’m wondering what amount of testing we want before we want to merge this change. If we wanted, we could run all tests twice, once with SHA-1 and once with SHA-256. This is something we do in gitoxide for the large majority of tests. gix-testtools, in particular, has support for both hashes, so we could also add gix-testtools as a dependency.

That said, though, I’m also fine with not adding much in terms of testing in this PR, knowing that test coverage in gitoxide at least is already quite thorough and assuming the same is true for git2. What do you think?