branchcache: store filtered hash and obsolete hash independently for V3
This will avoid the bug covered in tests/test-branches-obsolete.t when we stop
storing all heads explicitly in V3.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}