branchcache: gather new obsolete revision in a set
This is part of a series to more clearly split the update in two step. This
will allow us to introduce a fast path during update in a future changeset.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}