Use the database object format name but not read from git repoisitory everytime and fix possible migration wrong objectformat when migrating a sha256 repository (#29294)

Now we can get object format name from git command line or from the
database repository table. Assume the column is right, we don't need to
read from git command line every time.

This also fixed a possible bug that the object format is wrong when
migrating a sha256 repository from external.

<img width="658" alt="image"
src="https://github.com/go-gitea/gitea/assets/81045/6e9a9dcf-13bf-4267-928b-6bf2c2560423">

routers/private/hook_pre_receive.go

@@ -145,7 +145,7 @@ func preReceiveBranch(ctx *preReceiveContext, oldCommitID, newCommitID string, r
 
 	repo := ctx.Repo.Repository
 	gitRepo := ctx.Repo.GitRepo
-	objectFormat, _ := gitRepo.GetObjectFormat()
+	objectFormat := ctx.Repo.GetObjectFormat()
 
 	if branchName == repo.DefaultBranch && newCommitID == objectFormat.EmptyObjectID().String() {
 		log.Warn("Forbidden: Branch: %s is the default branch in %-v and cannot be deleted", branchName, repo)