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/HEAD

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

modules/git/tests/repos/repo6_merge_sha256/config

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

modules/git/tests/repos/repo6_merge_sha256/description

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

modules/git/tests/repos/repo6_merge_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_merge_sha256/info/refs

@@ -0,0 +1,4 @@
+d2e5609f630dd8db500f5298d05d16def282412e3e66ed68cc7d0833b29129a1	refs/heads/main
+b45258e9823233edea2d40d183742f29630e1e69300479fb4a55eabfe9b1d8bf	refs/heads/merge/add_file
+ff2b996e2fa366146300e4c9e51ccb6818147b360e46fa1437334f4a690955ce	refs/heads/merge/modify_file
+da1ded40dc8e5b7c564171f4bf2fc8370487decfb1cb6a99ef28f3ed73d09172	refs/heads/merge/remove_file

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

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

modules/git/tests/repos/repo6_merge_sha256/packed-refs

@@ -0,0 +1,5 @@
+# pack-refs with: peeled fully-peeled sorted 
+d2e5609f630dd8db500f5298d05d16def282412e3e66ed68cc7d0833b29129a1 refs/heads/main
+b45258e9823233edea2d40d183742f29630e1e69300479fb4a55eabfe9b1d8bf refs/heads/merge/add_file
+ff2b996e2fa366146300e4c9e51ccb6818147b360e46fa1437334f4a690955ce refs/heads/merge/modify_file
+da1ded40dc8e5b7c564171f4bf2fc8370487decfb1cb6a99ef28f3ed73d09172 refs/heads/merge/remove_file

modules/git/tests/repos/repo6_merge_sha256/refs/heads/main

@@ -0,0 +1 @@
+d2e5609f630dd8db500f5298d05d16def282412e3e66ed68cc7d0833b29129a1