Config: Fix git errors when .git does not exist
Summary: Bug: T16023
Test Plan: Delete `.git` and visit `/config/`
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16023
Differential Revision: https://we.phorge.it/D25931