* 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>

* only try to get HeadBranch if HeadRepo exist

* impruve

* no nil error

* add TEST

* correct error msg

* Move pull request api convert to convert package

* Rename ToPullRequest to ToAPIPullRequest