library: Fix git_ensure_remote() when remote isn't yet set up
library: Fix git_ensure_remote() when remote isn't yet set up

I was using `git show` to check if a remote is configured locally, but that
actually doesn't work as it happily shows info for anything you pass it.
Instead let's use `git config` to check that the remote configuration does
indeed exist.

ssssam/nightbus,ssssam/nightbus