branchmap: encapsulate cache updating in the map itself
Rather than have a repository update the cache, move handling of cache updates
into the branchmap module, in the form of a custom mapping class.
This makes later performance improvements easier to handle too.
Differential Revision: https://phab.mercurial-scm.org/D5638
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}