Abstract hash function usage (#28138)

Refactor Hash interfaces and centralize hash function. This will allow
easier introduction of different hash function later on.

This forms the "no-op" part of the SHA256 enablement patch.

services/repository/branch.go

@@ -377,6 +377,11 @@ func DeleteBranch(ctx context.Context, doer *user_model.User, repo *repo_model.R
 		return fmt.Errorf("GetBranch: %vc", err)
 	}
 
+	objectFormat, err := gitRepo.GetObjectFormat()
+	if err != nil {
+		return err
+	}
+
 	if rawBranch.IsDeleted {
 		return nil
 	}
@@ -403,7 +408,7 @@ func DeleteBranch(ctx context.Context, doer *user_model.User, repo *repo_model.R
 		&repo_module.PushUpdateOptions{
 			RefFullName:  git.RefNameFromBranch(branchName),
 			OldCommitID:  commit.ID.String(),
-			NewCommitID:  git.EmptySHA,
+			NewCommitID:  objectFormat.Empty().String(),
 			PusherID:     doer.ID,
 			PusherName:   doer.Name,
 			RepoUserName: repo.OwnerName,