Rework create/fork/adopt/generate repository to make sure resources will be cleanup once failed (#31035)

Fix #28144 

To make the resources will be cleanup once failed. All repository
operations now follow a consistent pattern:

- 1. Create a database record for the repository with the status
being_migrated.
- 2. Register a deferred cleanup function to delete the repository and
its related data if the operation fails.
- 3.	Perform the actual Git and database operations step by step.
- 4. Upon successful completion, update the repository’s status to
ready.

The adopt operation is a special case — if it fails, the repository on
disk should not be deleted.

models/git/branch.go

@@ -235,6 +235,11 @@ func GetDeletedBranchByID(ctx context.Context, repoID, branchID int64) (*Branch,
 	return &branch, nil
 }
 
+func DeleteRepoBranches(ctx context.Context, repoID int64) error {
+	_, err := db.GetEngine(ctx).Where("repo_id=?", repoID).Delete(new(Branch))
+	return err
+}
+
 func DeleteBranches(ctx context.Context, repoID, doerID int64, branchIDs []int64) error {
 	return db.WithTx(ctx, func(ctx context.Context) error {
 		branches := make([]*Branch, 0, len(branchIDs))