[API] get pull, return head branch sha, even if deleted (#14931)
[API] get pull, return head branch sha, even if deleted (#14931)

* API: return head branch sha, even if deleted

* relax if ref not resolvable
go-gitea/gitea,go-gitea/gitea,vdbt/gitea,cez81/gitea,cez81/gitea,vdbt/gitea,cez81/gitea,go-gitea/gitea