if rev == bookmarkrev:

                selected = n

        tags = []

        for tag, tagrev in repo.tags.iteritems():

            if tag == 'tip':

                continue

            n = 'tag:%s:%s' % (tag, tagrev)

            tags.append((n, tag))

            if rev == tagrev:

                selected = n

        # prio 1: rev was selected as existing entry above

        # prio 2: create special entry for rev; rev _must_ be used

        specials = []

        if rev and selected is None:

            selected = 'rev:%s:%s' % (rev, rev)

            specials = [(selected, '%s: %s' % (_("Changeset"), rev[:12]))]

        # prio 3: most recent peer branch

        if peers and not selected:

            selected = peers[0][0][0]

        # prio 4: tip revision

        if not selected:

            if h.is_hg(repo):

                if tipbranch:

                    selected = 'branch:%s:%s' % (tipbranch, tiprev)

                else:

                    selected = 'tag:null:0'

                    tags.append((selected, 'null'))

            else:

                if 'master' in repo.branches:

                    selected = 'branch:master:%s' % repo.branches['master']

                else:

                    k, v = repo.branches.items()[0]

                    selected = 'branch:%s:%s' % (k, v)

        groups = [(specials, _("Special")),

                  (peers, _("Peer branches")),

                  (bookmarks, _("Bookmarks")),

                  (branches, _("Branches")),

                  (tags, _("Tags")),

                  ]

        return [g for g in groups if g[0]], selected

    def _get_is_allowed_change_status(self, pull_request):

        owner = self.authuser.user_id == pull_request.user_id

        reviewer = self.authuser.user_id in [x.user_id for x in

                                                   pull_request.reviewers]

        return self.authuser.admin or owner or reviewer

    def _load_compare_data(self, pull_request, enable_comments=True):

        """

        Load context data needed for generating compare diff

        :param pull_request:

        """

        c.org_repo = pull_request.org_repo

        (c.org_ref_type,

         c.org_ref_name,

         c.org_rev) = pull_request.org_ref.split(':')

        c.other_repo = pull_request.other_repo

        (c.other_ref_type,

         c.other_ref_name,

         c.other_rev) = pull_request.other_ref.split(':') # other_rev is ancestor

        org_scm_instance = c.org_repo.scm_instance # property with expensive cache invalidation check!!!

        c.cs_repo = c.org_repo

        c.cs_ranges = [org_scm_instance.get_changeset(x) for x in pull_request.revisions]

        c.cs_ranges_org = None # not stored and not important and moving target - could be calculated ...

        revs = [ctx.revision for ctx in reversed(c.cs_ranges)]

        c.jsdata = json.dumps(graph_data(org_scm_instance, revs))

        c.available = []

        c.org_branch_name = c.org_ref_name

        other_scm_instance = c.other_repo.scm_instance

        if org_scm_instance.alias == 'hg' and c.other_ref_name != 'ancestor':

            if c.org_ref_type != 'branch':

                c.org_branch_name = org_scm_instance.get_changeset(c.org_ref_name).branch # use ref_type ?

            other_branch_name = c.other_ref_name

            if c.other_ref_type != 'branch':

                other_branch_name = other_scm_instance.get_changeset(c.other_ref_name).branch # use ref_type ?

            # 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, other_branch_name):

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

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

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

                                                    revs[0], c.org_branch_name, revs[0])

                if arevs:

                    if c.pull_request.is_closed():

                        c.update_msg = _('This pull request has been closed and can not be updated with descendent changes on %s:') % c.org_branch_name