bundle2: make the "hgtagsfnodes" part advisory
This bundle2 part is about helping the client to warms its cache. There is no
reason for it to be mandatory.
So we mark it advisory.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}