test: use a more verbose output in the test While debugging some test failure, I released the test never checks if the relevant changesets were preserved. So I am updating the test from `hg parents` usage to `hg log -G` with a special template. This increase the area covered by the test and clarify the test failures.

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