chore: add github git auth helper

Adds the auth helper to the config. This is used mainly for sapling so
we can push and pull to git repos. I need to run this after each time I
configure the system because this file is a template not a symlink

This was originally added with the `gh` cli tool running the following
command:

```bash
gh auth setup-git
```

site-modules/core/templates/gitconfig.liquid

@@ -40,3 +40,9 @@
   file-added-label = "added:"
   file-renamed-label = "renamed:"
 
+[credential "https://github.com"]
+  helper =
+  helper = !/usr/bin/gh auth git-credential
+[credential "https://gist.github.com"]
+  helper =
+  helper = !/usr/bin/gh auth git-credential