perffncachewrite: load fncache after lock is acquired
Without this patch, running perffncachewrite on a repository destroy its
fncache.
Lock Acquisition drops various caches, including the fncache one. Then writing
of an non-loaded fncache result into an empty one.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}