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.
This adds a thread-safe RoundTripper that can be used for mocking HTTP
requests in tests. Incoming requests are matched based on their
contents, not the order the stubs were registered in.
