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.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown