Add SHA256 Support

[nick]

I am a satisfied user of Cgit, but I wish it supported SHA-256 repos.

Gitea will soon support SHA-256:

https://github.com/go-gitea/gitea/pull/23894

There is also at least one repository hosting platform which supports
SHA-256:

https://sha2git.com/

According to Git documentation, it should be safe to implement:

https://git-scm.com/docs/git-init#Documentation/git-init.txt---object-formatltformatgt

"Historically, we warned that SHA-256 repositories may later need
backward incompatible changes when we introduce such interoperability
features. Today, we only expect compatible changes. Furthermore, if such
changes prove to be necessary, it can be expected that SHA-256
repositories created with today’s Git will be usable by future
versions of Git without data loss."