branchcache: simplify the branch rev cache test
We don't need that many content dump and this gets in the way in change in
access pattern (e.g. accessing revision in a different order change the order of
branches in the "names" file).
So we simplify this test in advance.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}