narrow: add capabilities for local repos, not just remote peers
This fixes the bug where running `hg clone --narrow ./local-repo`
fails with
abort: server does not support narrow clones
even when the server has narrow enabled.
Differential Revision: https://phab.mercurial-scm.org/D10357
#require docutils gettext
$ $TESTDIR/check-gendoc da
checking for parse errors