Bug 1863361 - Use --root argument to mach taskgraph to find VCS root when running `taskgraph diff`. r=taskgraph-reviewers,ahal

Differential Revision: https://phabricator.services.mozilla.com/D192863

taskcluster/gecko_taskgraph/main.py

@@ -372,7 +372,11 @@ def show_taskgraph(options):
     output_file = options["output_file"]
 
     if options["diff"]:
-        repo = get_repository(os.getcwd())
+        # --root argument is taskgraph's config at <repo>/taskcluster/ci
+        repo_root = os.getcwd()
+        if options["root"]:
+            repo_root = f"{options['root']}/../.."
+        repo = get_repository(repo_root)
 
         if not repo.working_directory_clean():
             print(