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
$ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
Mercurial Distributed SCM (version *) (glob)