First, consolidate the functionality between `pr merge` and `pr
checkout` that resolves the default branch name of the base repo. With
an added bonus, the new approach avoids an API request when one isn't
necessary.
Then, ensure that checking out 3rd-party PRs will result in local branch
name such as `<owner>/<branch>` when the head branch of the repository
matches the default branch of the base repository. We already have had
code in place to take care of this, but it only took effect in the `pr
checkout <number>`-style invocation.
Previously, the repository owner+name component of the URL was ignored
and only the pull request number was read. Now, the URL dictates which
base repository will be used.
This stubs stderr separately from stdout in command tests (before those
streams were combined) and improves test assertions around output.
Additionally, no longer use the `cmd.Print*()` family of Cobra functions
because their name sounds like the text will go to stdout, but they
write to stderr instead. Use the more explicit `cmd.ErrOrStderr()` as
output destination instead.
