When an existing `headRepo` couldn't be detected, it's time to auto-fork
one. Unfortunately, an obscure Go behavior made it seem like `headRepo`
was a non-nil value, where in fact it did contain a nil pointer which
would crash the process.
This avoids ever assigning nil pointers to `var headRepo ghrepo.Interface`.
This code was put in place in preparation for a feature that never
shipped. Namely, we wanted to use the commit hash for the base branch so
we can get an accurate `git log` involving the changes in a pull
request. However, getting the commit hash from API is not the way to go
because the latest commit might not be available in the person's local
repository, and using a local tracking branch for base such as
`origin/master` works quite well in most cases without dereferencing it.
Now passing base repo as reference to `prSelectorForCurrentBranch()` so
that it doesn't have to (inexactly) determine the base repo itself and
risk it being different than the base repo determined during `pr
status/view`.
