On first run in a git repository, `BaseRepo()` will now prompt the user
which repository should be queried as base repository if there are
multiple git remotes or when we are in the context of a fork.
In non-interactive mode, the prompt is skipped and we default to the
first git remote instead.
After the base repo is resolved, the result is cached in the local
repository using `git config` so that RepositoryNetwork API lookups can
be avoided in the future.
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
