changegroup: always use the treemanifest-enabled version of _packmanifests()
It works for flat manifests too. We just cannot use cg1 or cg2 if we
have subdirectory manifests.
Differential Revision: https://phab.mercurial-scm.org/D4124
#require docutils gettext $ $TESTDIR/check-gendoc zh_TW checking for parse errors