pull-requests: don't select first commit in case we don't get default branch. Loading

compare from commit 0 to something selected is very heavy to compute. Instead force
user to select the ref manually. ref  #5414

rhodecode/model/pull_request.py

@@ -1426,8 +1426,8 b' class PullRequestModel(BaseModel):'
                     repo.branches[repo.DEFAULT_BRANCH_NAME]
                 )
             elif repo.commit_ids:
-                rev = repo.commit_ids[0]
-                selected = 'rev:%s:%s' % (rev, rev)
+                # make the user select in this case
+                selected = None
             else:
                 raise EmptyRepositoryError()
         return groups, selected