phases: use revision number in `_pushdiscoveryphase` We now reach our target checkpoint in terms of rev-num conversion. The `_pushdiscoveryphase` function is now performing graph computation based on revision number only. Avoiding repeated conversion from node-id to rev-num. See previous changeset updated `new_heads` for rationnal. Again, time saved in the 100 milliseconds order of magnitude for the mozilla-try benchmark I have been using. However, wow that the logic is done using revision number, we can look into having better logic in the next changesets, which will provide a much bigger speedup.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "phabricator.uri": "https://phab.mercurial-scm.org/",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}