This completely rewrites the PR lookup mechanism so that the caller
must specify the GraphQL fields to query for each PR. Additionally, this
fixes some export problems with `pr view --json`.
Features:
- Each pr command now gets assigned a concept of a Finder. This makes it
easier to stub the PR in tests without having to stub the underlying
HTTP calls or git invocations.
- `pr view --web` is much faster since it only fetches the "url" field.
- `pr diff 123` now skips a whole API call where a whole PR was
unnecessarily preloaded just to access its diff in a subsequent call.
- PullRequestGraphQL query builder is now used to construct queries.
- A bunch of individual commands are now freed of having to know about
concepts such as BaseRepo, Branch, Config, or Remotes.
This removes sensitivity to the BROWSER environment variable in tests
and makes it easier to verify the URL that the browser was invoked with
without having to stub sub-processes.
