Fix bug when deleting a migrated branch (#32075)

After migrating a repository with pull request, the branch is missed and
after the pull request merged, the branch cannot be deleted.

services/repository/branch.go

@@ -483,22 +483,23 @@ func DeleteBranch(ctx context.Context, doer *user_model.User, repo *repo_model.R
 	}
 
 	rawBranch, err := git_model.GetBranch(ctx, repo.ID, branchName)
-	if err != nil {
+	if err != nil && !git_model.IsErrBranchNotExist(err) {
 		return fmt.Errorf("GetBranch: %vc", err)
 	}
 
-	if rawBranch.IsDeleted {
-		return nil
-	}
+	// database branch record not exist or it's a deleted branch
+	notExist := git_model.IsErrBranchNotExist(err) || rawBranch.IsDeleted
 
 	commit, err := gitRepo.GetBranchCommit(branchName)
 	if err != nil {
 		return err
 	}
 
 	if err := db.WithTx(ctx, func(ctx context.Context) error {
-		if err := git_model.AddDeletedBranch(ctx, repo.ID, branchName, doer.ID); err != nil {
-			return err
+		if !notExist {
+			if err := git_model.AddDeletedBranch(ctx, repo.ID, branchName, doer.ID); err != nil {
+				return err
+			}
 		}
 
 		return gitRepo.DeleteBranch(branchName, git.DeleteBranchOptions{