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

services/pull/pull.go

@@ -328,7 +328,7 @@ func AddTestPullRequestTask(doer *user_model.User, repoID int64, branch string,
 			if err == nil {
 				for _, pr := range prs {
 					objectFormat, _ := git.GetObjectFormatOfRepo(ctx, pr.BaseRepo.RepoPath())
-					if newCommitID != "" && newCommitID != objectFormat.Empty().String() {
+					if newCommitID != "" && newCommitID != objectFormat.EmptyObjectID().String() {
 						changed, err := checkIfPRContentChanged(ctx, pr, oldCommitID, newCommitID)
 						if err != nil {
 							log.Error("checkIfPRContentChanged: %v", err)