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.
For operations such as closing an issue or merging a PR, we would
display the success icon (a checkmark) in red and magenta colors,
respectively, to reflect the latest state of the record operated on
(red: closed; magenta: merged).
This was always confusing to me, seeing it both in code and in the UI,
because I'm instinctively thinking that it's a bug and have to remind
myself that it's by design.
