Add checkbox to delete pull branch after successful merge (#16049)

Add checkbox to delete pull branch after successful merge (#16049)

* Add checkbox to delete pull branch after successful merge

* Omit DeleteBranchAfterMerge field in json

* Log a warning instead of error when PR head branch deleted

* Add DefaultDeleteBranchAfterMerge to PullRequestConfig

* Add support for delete_branch_after_merge via API

* Fix for API: the branch should be deleted from the HEAD repo

If head and base repo are the same, reuse the already opened ctx.Repo.GitRepo

* Don't delegate to CleanupBranch, only reuse branch deletion code

CleanupBranch contains too much logic that has already been performed by the Merge

* Reuse gitrepo in MergePullRequest

Co-authored-by: Andrew Thornton <art27@cantab.net>