Allow `git status` to detect when upstream is out of date

[joshlf]

Currently, regardless of whether we've updated GitHub/made local changes, we get:

$ git status
On branch read-only
Your branch is up to date with 'read-only'.

nothing to commit, working tree clean

This is as a result of the fact that, for private branches, git thinks that the local repo is the upstream.

Gemini design.