if branch_rev:

            branch_rev = safe_str(branch_rev)

            # a revset not restricting to merge() would be better

            # (especially because it would get the branch point)

            # ... but is currently too expensive

            # including branches of children could be nice too

            peerbranches = set()

            for i in repo._repo.revs(

                "sort(parents(branch(id(%s)) and merge()) - branch(id(%s)), -rev)",

                branch_rev, branch_rev):

                abranch = repo.get_changeset(i).branch

                if abranch not in peerbranches:

                    n = 'branch:%s:%s' % (abranch, repo.get_changeset(abranch).raw_id)

                    peers.append((n, abranch))

                    peerbranches.add(abranch)

        selected = None

        tiprev = repo.tags.get('tip')

        tipbranch = None

        branches = []

        for abranch, branchrev in repo.branches.iteritems():

            n = 'branch:%s:%s' % (abranch, branchrev)

            desc = abranch

            if branchrev == tiprev:

                tipbranch = abranch

                desc = '%s (tip)' % desc

            branches.append((n, desc))

            if rev == branchrev:

                selected = n

            if branch == abranch:

                if not rev:

                    selected = n

                branch = None

        if branch:  # branch not in list - it is probably closed

            branchrev = repo.closed_branches.get(branch)

            if branchrev:

                n = 'branch:%s:%s' % (branch, branchrev)

                branches.append((n, _('%s (closed)') % branch))

                selected = n

                branch = None

            if branch:

                log.error('branch %r not found in %s', branch, repo)

        bookmarks = []

        for bookmark, bookmarkrev in repo.bookmarks.iteritems():

            n = 'book:%s:%s' % (bookmark, bookmarkrev)

            bookmarks.append((n, bookmark))

            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

                    else:

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

                    c.available = [org_scm_instance.get_changeset(x) for x in arevs]

                else:

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

            if org_scm_instance._repo.revs('head() & not (%s::) & branch(%s)', revs[0], c.org_branch_name):

                c.update_msg_other = _('Note: Branch %s also contains unrelated changes.') % c.org_branch_name

            else:

                c.update_msg_other = _('Branch %s does not contain other changes.') % c.org_branch_name

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

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

        c.statuses = c.org_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

        log.debug('running diff between %s and %s in %s'

                  % (c.other_rev, c.org_rev, org_scm_instance.path))

        txtdiff = org_scm_instance.get_diff(rev1=safe_str(c.other_rev), rev2=safe_str(c.org_rev),

                                      ignore_whitespace=ignore_whitespace,

                                      context=line_context)

        diff_processor = diffs.DiffProcessor(txtdiff or '', format='gitdiff',

                                             diff_limit=diff_limit)

        _parsed = diff_processor.prepare()

        c.limited_diff = False

        if isinstance(_parsed, LimitedDiffContainer):

            c.limited_diff = True

        c.files = []

        c.changes = {}

        c.lines_added = 0

        c.lines_deleted = 0

        for f in _parsed:

            st = f['stats']

            c.lines_added += st['added']

            c.lines_deleted += st['deleted']

            fid = h.FID('', f['filename'])

            c.files.append([fid, f['operation'], f['filename'], f['stats']])