fix: prioritize --branch flag over default behavior

Author: iseki0

cmd/murphy/internal/scan/scan.go

@@ -155,7 +155,6 @@ func scan(ctx context.Context, dir string, accessType model.AccessType, mode mod
 	createSubtask.PackagePrivateName = privateSourceName
 	createSubtask.ProjectTagNames = projectTagNames
 	createSubtask.IsAutonomous = scanCodeHash
-	createSubtask.Branch = &branch
 	if createSubtask.ProjectTagNames == nil {
 		createSubtask.ProjectTagNames = make([]string, 0)
 	}
@@ -180,6 +179,9 @@ func scan(ctx context.Context, dir string, accessType model.AccessType, mode mod
 	} else {
 		assignGitInfoToCreateSubtaskReq(&createSubtask, gitSummary)
 	}
+	if branch != "" {
+		createSubtask.Branch = &branch
+	}
 
 	// call API
 	createTaskResp, e := api.CreateSubTask(api.DefaultClient(), &createSubtask)