When the git ref is unable to be found return broken pr (#13218)

Fix #13216

Signed-off-by: Andrew Thornton <art27@cantab.net>

routers/repo/pull.go

@@ -311,7 +311,7 @@ func PrepareMergedViewPullInfo(ctx *context.Context, issue *models.Issue) *git.C
 	compareInfo, err := ctx.Repo.GitRepo.GetCompareInfo(ctx.Repo.Repository.RepoPath(),
 		pull.MergeBase, pull.GetGitRefName())
 	if err != nil {
-		if strings.Contains(err.Error(), "fatal: Not a valid object name") {
+		if strings.Contains(err.Error(), "fatal: Not a valid object name") || strings.Contains(err.Error(), "unknown revision or path not in the working tree") {
 			ctx.Data["IsPullRequestBroken"] = true
 			ctx.Data["BaseTarget"] = pull.BaseBranch
 			ctx.Data["NumCommits"] = 0