hg: remove reserved identifiers
Per 7.1.3 paragraph, from C draft:
All identifiers that begin with an underscore and either an uppercase letter or
another underscore are always reserved for any use.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}