[spike] Automatically detect the PR branch the user is on

[jcansdale]

The PR associated with the current branch is only highlighted when the user created or opened the PR using GitHub for Visual Studio. This isn't very helpful for users who are new to the extension.

What this PR does

This spike attempts to look up the PR associated with the current branch if the branch hasn't already been tagged. It uses the simple approach of matching refs/heads/<name> references with refs/pull/xxx/head references. This approach has worked well for https://github.com/jcansdale/GitPullRequest.

Challenges

There are issues with spam PRs, where an external user creates a PR from an existing branch in the target repository. For example, a user might create a PR from the master branch in a repository. The owner of the repository is likely to close the spam PR, but won't delete the master branch! Unfortunately the above approach can detect deleted branches but no closed PRs.

Todo

Implement this using the GraphQL API so we can detect and ignore closed PRs.