largefiles: avoid walking full manifest
When using treemanifest, diffs between two manifests can often be much
cheaper than iterating the full manifests (because common subtrees are
skipped).
Differential Revision: https://phab.mercurial-scm.org/D5644
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}