manifest: rename manifestlog._treeinmem to ._treemanifests
Not sure what "inmem" was supposed to indicate. This object is an
interface to manifest data on disk as well as "in memory" (assuming
that's what "inmem" means).
Differential Revision: https://phab.mercurial-scm.org/D4278
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}