transaction: split new files into a separate set
Journal entries with size 0 are common as they represent new revlog
files. Move them from the dictionary into a set as the former is more
dense. This reduces peak RSS by 70MB for the NetBSD test repository with
around 450k files under .hg/store.
Differential Revision: https://phab.mercurial-scm.org/D9278
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}