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>

models/repo/repo.go

@@ -180,7 +180,7 @@ type Repository struct {
 	IsFsckEnabled                   bool               `xorm:"NOT NULL DEFAULT true"`
 	CloseIssuesViaCommitInAnyBranch bool               `xorm:"NOT NULL DEFAULT false"`
 	Topics                          []string           `xorm:"TEXT JSON"`
-	ObjectFormatName                string             `xorm:"-"`
+	ObjectFormatName                string             `xorm:"VARCHAR(6) NOT NULL DEFAULT 'sha1'"`
 
 	TrustModel TrustModelType
 
@@ -276,10 +276,6 @@ func (repo *Repository) AfterLoad() {
 	repo.NumOpenMilestones = repo.NumMilestones - repo.NumClosedMilestones
 	repo.NumOpenProjects = repo.NumProjects - repo.NumClosedProjects
 	repo.NumOpenActionRuns = repo.NumActionRuns - repo.NumClosedActionRuns
-
-	// this is a temporary behaviour to support old repos, next step is to store the object format in the database
-	// and read from database so this line could be removed. To not depend on git module, we use a constant variable here
-	repo.ObjectFormatName = "sha1"
 }
 
 // LoadAttributes loads attributes of the repository.