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

* API: return head branch sha, even if deleted

* relax if ref not resolvable

modules/convert/pull.go

@@ -134,6 +134,24 @@ func ToAPIPullRequest(pr *models.PullRequest) *api.PullRequest {
 		}
 	}
 
+	if len(apiPullRequest.Head.Sha) == 0 && len(apiPullRequest.Head.Ref) != 0 {
+		baseGitRepo, err := git.OpenRepository(pr.BaseRepo.RepoPath())
+		if err != nil {
+			log.Error("OpenRepository[%s]: %v", pr.BaseRepo.RepoPath(), err)
+			return nil
+		}
+		defer baseGitRepo.Close()
+		refs, err := baseGitRepo.GetRefsFiltered(apiPullRequest.Head.Ref)
+		if err != nil {
+			log.Error("GetRefsFiltered[%s]: %v", apiPullRequest.Head.Ref, err)
+			return nil
+		} else if len(refs) == 0 {
+			log.Error("unable to resolve PR head ref")
+		} else {
+			apiPullRequest.Head.Sha = refs[0].Object.String()
+		}
+	}
+
 	if pr.Status != models.PullRequestStatusChecking {
 		mergeable := !(pr.Status == models.PullRequestStatusConflict || pr.Status == models.PullRequestStatusError) && !pr.IsWorkInProgress()
 		apiPullRequest.Mergeable = mergeable