This API was just fixed in github.com, but it will take a while for the
change to propagate to GitHub Enterprise installs, so guard ourselves
from false positives when querying forks.
Before: the default push target for the current branch in `pr create`
was the first repository found among git remotes that has write access.
Now: the default push target is the fork the base repo, if said fork
exists and has write access, falling back to old behavior otherwise.
This change in the default is to facilitate contributions to projects
that have a hard requirement that all pull requests (even those opened
by people with write access to that project) come from forks.
Also define a handful of utility methods:
- `New(owner, repo)`
- `FullName`: the name slash owner pair
- `FromFullName`: parse the name slash owner pair
- `FromURL`: parse a GitHub.com URL
- `IsSame(r1, r2)`: compare two repositories
