convert: add missing numcommits() override to hg sources
Otherwise, the progressbar in converter.walktree() gets None as its total during
the initial scan. (Though that seems harmless, and there are other foreign vcs
sources without this.)
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}