Resolve "gdk doctor errors outside of GDK.root"

What does this Merge Request do?

This MR ensures gdk doctor works anywhere within the GDK.root and beyond. It also:

• Updates the 'An update for GDK is available.' diagnostic to ensure we only display the detail if the branch is behind origin/master
• Adds the config.gitlab.dir path setting

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• [-] Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.

Closes #871 (closed)