branchcache: move the processing of the new data in a dedicated method
In a future changeset, this will allow the V3 of the branch cache to use a fast
path when possible.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}