[upstream_utils] Use git to get repo root (#6967)

upstream_utils/upstream_utils.py

@@ -65,12 +65,12 @@ def get_repo_root():
 
     An empty string is returned if no repository root was found.
     """
-    current_dir = os.path.abspath(os.getcwd())
-    while current_dir != os.path.dirname(current_dir):
-        if os.path.exists(current_dir + os.sep + ".git"):
-            return current_dir
-        current_dir = os.path.dirname(current_dir)
-    return ""
+    return subprocess.run(
+        ["git", "rev-parse", "--show-toplevel"],
+        stdout=subprocess.PIPE,
+        stderr=subprocess.DEVNULL,
+        encoding="ascii",
+    ).stdout.rstrip()
 
 
 def walk_if(top, pred):