dirstate-v2: fix upgrade on an empty repository
This used to crash as the dirstate file does not exist in this case.
Differential Revision: https://phab.mercurial-scm.org/D11866
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}