:param repo: repo instance

        """

        rev = ref[1] # default and used for git

        if repo.scm_instance.alias == 'hg':

            # lookup up the exact node id

            _revset_predicates = {

                    'branch': 'branch',

                    'book': 'bookmark',

                    'tag': 'tag',

                    'rev': 'id',

                }

            rev_spec = "max(%s(%%s))" % _revset_predicates[ref[0]]

            revs = repo.scm_instance._repo.revs(rev_spec, safe_str(ref[1]))

            if revs:

                rev = revs[-1]

            # else: TODO: just report 'not found'

        try:

            return repo.scm_instance.get_changeset(rev).raw_id

        except EmptyRepositoryError, e:

            if not redirect_after:

                return None

            h.flash(h.literal(_('There are no changesets yet')),

                    category='warning')

            redirect(url('summary_home', repo_name=repo.repo_name))

        except RepositoryError, e:

            log.error(traceback.format_exc())

            h.flash(str(e), category='warning')

            if not partial:

                redirect(h.url('summary_home', repo_name=repo.repo_name))

            raise HTTPBadRequest()

    def _get_changesets(self, alias, org_repo, org_ref, other_repo, other_ref, merge):

        """

        Returns a list of changesets that can be merged from org_repo@org_ref

        to other_repo@other_ref ... and the ancestor that would be used for merge

        :param org_repo:

        :param org_ref:

        :param other_repo:

        :param other_ref:

        :param tmp:

        """

        ancestor = None

            # lookup up the exact node id

            _revset_predicates = {

                    'branch': 'branch',

                    'book': 'bookmark',

                    'tag': 'tag',

                    'rev': 'id',

                }

            org_rev_spec = "max(%s(%%s))" % _revset_predicates[org_ref[0]]

            org_revs = org_repo._repo.revs(org_rev_spec, safe_str(org_ref[1]))

            org_rev = org_repo._repo[org_revs[-1] if org_revs else -1].hex()

            other_revs_spec = "max(%s(%%s))" % _revset_predicates[other_ref[0]]

            other_revs = other_repo._repo.revs(other_revs_spec, safe_str(other_ref[1]))

            other_rev = other_repo._repo[other_revs[-1] if other_revs else -1].hex()

            #case two independent repos

            if org_repo != other_repo:

                hgrepo = unionrepo.unionrepository(other_repo.baseui,

                                                   other_repo.path,

                                                   org_repo.path)

                # all the changesets we are looking for will be in other_repo,

                # so rev numbers from hgrepo can be used in other_repo

            #no remote compare do it on the same repository

            else:

                hgrepo = other_repo._repo

            if merge:

                revs = hgrepo.revs("ancestors(id(%s)) and not ancestors(id(%s)) and not id(%s)",

                                   other_rev, org_rev, org_rev)

                ancestors = hgrepo.revs("ancestor(id(%s), id(%s))", org_rev, other_rev)

                if ancestors:

                    # pick arbitrary ancestor - but there is usually only one

                    ancestor = hgrepo[ancestors[0]].hex()

            else:

                # TODO: have both + and - changesets

                revs = hgrepo.revs("id(%s) :: id(%s) - id(%s)",

                                   org_rev, other_rev, org_rev)

            changesets = [other_repo.get_changeset(rev) for rev in revs]

        elif alias == 'git':

            if org_repo != other_repo:

                raise Exception('Comparing of different GIT repositories is not'

                                'allowed. Got %s != %s' % (org_repo, other_repo))