[update-checkout] Use basename to get repo name for Git failures

`os.path.dirname` removes the last path component of a path. If there was a git failure in `~/src/swift`, this means that we show `~/src` as the repo name, which isn’t very helpful. Use `os.path.basename`, which returns the last path component, ie. `swift`.

Author: ahoppen

utils/update_checkout/update_checkout/git_command.py

@@ -78,7 +78,7 @@ def run(
                     f"status {str(e.returncode)}, aborting"
                 )
             raise GitException(
-                e.returncode, command, os.path.dirname(repo_path), output
+                e.returncode, command, os.path.basename(repo_path), output
             )
         except OSError as e:
             if fatal: