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.
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2