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_blame_sha256/HEAD

@@ -0,0 +1 @@
+ref: refs/heads/main

modules/git/tests/repos/repo6_blame_sha256/config

@@ -0,0 +1,6 @@
+[core]
+	repositoryformatversion = 1
+	filemode = true
+	bare = true
+[extensions]
+	objectformat = sha256

modules/git/tests/repos/repo6_blame_sha256/description

@@ -0,0 +1 @@
+Unnamed repository; edit this file 'description' to name the repository.

modules/git/tests/repos/repo6_blame_sha256/info/exclude

@@ -0,0 +1,6 @@
+# git ls-files --others --exclude-from=.git/info/exclude
+# Lines that start with '#' are comments.
+# For a project mostly in C, the following would be a good set of
+# exclude patterns (uncomment them if you want to use them):
+# *.[oa]
+# *~

modules/git/tests/repos/repo6_blame_sha256/info/refs

@@ -0,0 +1 @@
+e2f5660e15159082902960af0ed74fc144921d2b0c80e069361853b3ece29ba3	refs/heads/main

modules/git/tests/repos/repo6_blame_sha256/objects/info/packs

@@ -0,0 +1,2 @@
+P pack-fcb8a221b76025fd8415d3c562b611ac24312a5ffc3d3703d7c5cc906bdaee8e.pack
+

modules/git/tests/repos/repo6_blame_sha256/packed-refs

@@ -0,0 +1,2 @@
+# pack-refs with: peeled fully-peeled sorted 
+e2f5660e15159082902960af0ed74fc144921d2b0c80e069361853b3ece29ba3 refs/heads/main

modules/git/tests/repos/repo6_blame_sha256/refs/refs/main

@@ -0,0 +1 @@
+e2f5660e15159082902960af0ed74fc144921d2b0c80e069361853b3ece29ba3