Adjust object format interface (#28469)

- Remove `ObjectFormatID`
- Remove function `ObjectFormatFromID`.
- Use `Sha1ObjectFormat` directly but not a pointer because it's an
empty struct.
- Store `ObjectFormatName` in `repository` struct

routers/private/hook_verification.go

@@ -30,7 +30,7 @@ func verifyCommits(oldCommitID, newCommitID string, repo *git.Repository, env []
 
 	var command *git.Command
 	objectFormat, _ := repo.GetObjectFormat()
-	if oldCommitID == objectFormat.Empty().String() {
+	if oldCommitID == objectFormat.EmptyObjectID().String() {
 		// When creating a new branch, the oldCommitID is empty, by using "newCommitID --not --all":
 		// List commits that are reachable by following the newCommitID, exclude "all" existing heads/tags commits
 		// So, it only lists the new commits received, doesn't list the commits already present in the receiving repository