branch: allow changing branch of merge commits with --rev
Tests show that changing branch of merge commits works fine with evolution and
without, so let's allow it. Other safeguards should prevent users from shooting
themselves in the foot.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}