except EmptyRepositoryError:

                    c.a_branch_name = 'null' # not a branch name ... but close enough

            # candidates: descendants of old head that are on the right branch

            #             and not are the old head itself ...

            #             and nothing at all if old head is a descendent of target ref name

            if other_scm_instance._repo.revs('present(%s)::&%s', c.cs_ranges[-1].raw_id, c.a_branch_name):

                c.update_msg = _('This pull request has already been merged to %s.') % c.a_branch_name

            elif c.pull_request.is_closed():

                c.update_msg = _('This pull request has been closed and can not be updated.')

            else: # look for descendants of PR head on source branch in org repo

                avail_revs = org_scm_instance._repo.revs('%s:: & branch(%s)',

                                                         revs[0], c.cs_branch_name)

                if len(avail_revs) > 1: # more than just revs[0]

                    # also show changesets that not are descendants but would be merged in

                    show = set(org_scm_instance._repo.revs('::%ld & !::%s & !::%s',

                                                           avail_revs, revs[0], c.a_branch_name))

                    c.update_msg = _('This pull request can be updated with changes on %s:') % c.cs_branch_name

                else:

                    show = set()

                    c.update_msg = _('No changesets found for updating this pull request.')

                # TODO: handle branch heads that not are tip-most

                brevs = org_scm_instance._repo.revs('%s - %ld', c.cs_branch_name, avail_revs)

                if brevs:

                    c.update_msg_other = _('Note: Branch %s has another head: %s.') % (c.cs_branch_name,

                        h.short_id(org_scm_instance.get_changeset((max(brevs))).raw_id))

                avail_show = sorted(show, reverse=True)

        elif org_scm_instance.alias == 'git':

            c.update_msg = _("Git pull requests don't support updates yet.")

        c.avail_revs = avail_revs

        c.avail_cs = [org_scm_instance.get_changeset(r) for r in avail_show]

        c.avail_jsdata = json.dumps(graph_data(org_scm_instance, avail_show))

        raw_ids = [x.raw_id for x in c.cs_ranges]

        c.cs_comments = c.cs_repo.get_comments(raw_ids)

        c.statuses = c.cs_repo.statuses(raw_ids)

        ignore_whitespace = request.GET.get('ignorews') == '1'

        line_context = request.GET.get('context', 3)

        c.ignorews_url = _ignorews_url

        c.context_url = _context_url

        c.fulldiff = request.GET.get('fulldiff')

        diff_limit = self.cut_off_limit if not c.fulldiff else None

        # we swap org/other ref since we run a simple diff on one repo