path: add a method to retrieve a "push variant" of a path
This gets you the same path, but using the `pushurl` as destination.
This opens the way for a lot of different improvements, the one which
interests us is having `peer` objects aware of the `path` they came from.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}