Display pull request head branch even the branch deleted or repository deleted (#10413)

* 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>

modules/convert/pull_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 
 	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/structs"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -15,12 +16,19 @@ import (
 func TestPullRequest_APIFormat(t *testing.T) {
 	//with HeadRepo
 	assert.NoError(t, models.PrepareTestDatabase())
+	headRepo := models.AssertExistsAndLoadBean(t, &models.Repository{ID: 1}).(*models.Repository)
 	pr := models.AssertExistsAndLoadBean(t, &models.PullRequest{ID: 1}).(*models.PullRequest)
 	assert.NoError(t, pr.LoadAttributes())
 	assert.NoError(t, pr.LoadIssue())
 	apiPullRequest := ToAPIPullRequest(pr)
 	assert.NotNil(t, apiPullRequest)
-	assert.Nil(t, apiPullRequest.Head)
+	assert.EqualValues(t, &structs.PRBranchInfo{
+		Name:       "branch1",
+		Ref:        "refs/pull/2/head",
+		Sha:        "4a357436d925b5c974181ff12a994538ddc5a269",
+		RepoID:     1,
+		Repository: headRepo.APIFormat(models.AccessModeNone),
+	}, apiPullRequest.Head)
 
 	//withOut HeadRepo
 	pr = models.AssertExistsAndLoadBean(t, &models.PullRequest{ID: 1}).(*models.PullRequest)