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.
If the `--repo` flag is specified, then the user intends to select a
repository other than the current one. In that case, it doesn't make
sense to fall back to detecting the PR belonging to the current branch,
so throw a descriptive error instead.
