Prior to this change, the checks/git.fish test would fail if run from a
git interactive rebase (such as via `git rebase -i --exec 'ninja test'`),
because git itself would inject stuff into the environment. Teach the git
test how to clean up its environment first before running.
This makes the fish_git_prompt variable handlers kick in, meaning we
see the informative chars.
The big question here is what happens if there's a non-UTF-8 locale in
the test.
Theoretically we set LC_CTYPE, but.....
