Add support for sha256 repositories (#23894)

Currently only SHA1 repositories are supported by Gitea. This adds
support for alternate SHA256 with the additional aim of easier support
for additional hash types in the future.

Fixes: #13794
Limited by: https://github.com/go-git/go-git/issues/899
Depend on: #28138

<img width="776" alt="图片" src="https://github.com/go-gitea/gitea/assets/81045/5448c9a7-608e-4341-a149-5dd0069c9447">

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: 6543 <6543@obermui.de>

modules/git/tests/repos/repo6_merge_sha256/objects/info/packs

@@ -0,0 +1,3 @@
+P pack-2fff0848f8d8eab8f7902ac91ab6a096c7530f577d5c0a79c63d9ac2b44f7510.pack
+P pack-65162b86afdbac3c566696d487e67bb2a4a5501ca1fa3528fad8a9474fba7e50.pack
+