* Display pull request head branch even the branch deleted or repository deleted
* Merge getHeadRepo/loadHeadRepo and getBaseRepo/loadBaseRepo on pull and fill repo when pr.Issue.Repo is available
* retrieve sha from pull head when pull request branch deleted and fix tests
* Fix test
* Ensure MustHeadRepoName returns empty string if no head repo
Co-authored-by: zeripath <art27@cantab.net>