When reviewing pull requests, instead of having to `git clone` the repo and checkout the source branch, it would be easier to check out a pull request reference which checks out the branch in one step.

Note: this is different from a pull request 'merge' ref which would merge the latest from the source branch in a pull request to the destination branch, giving a preview of the code merged. That request is tracked in https://jira.atlassian.com/browse/BCLOUD-5814.