The appropriate name for the remote pointing at the repo we maintain
may be `upstream` for most of our repos... but not when we're
downstream of someone else, e.g. for react-native. So, make it easy
to configure per-repo.
Minor fixes that enable the ability to:
- Re-run fetch-rebase-pull-request.
- Specify the name of the remote repo as an optional second parameter.
The default remains 'upstream'.