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.

contrib/perf.py

@@ -980,9 +980,10 b' def perffncacheload(ui, repo, **opts):'
 def perffncachewrite(ui, repo, **opts):
     timer, fm = gettimer(ui, opts)
     s = repo.store
+    lock = repo.lock()
     s.fncache._load()
-    lock = repo.lock()
     tr = repo.transaction('perffncachewrite')
+    tr.addbackup('fncache')
     def d():
         s.fncache._dirty = True
         s.fncache.write(tr)

tests/test-contrib-perf.t

@@ -146,7 +146,11 b' perfstatus'
 #if repofncache
   $ hg perffncacheencode
   $ hg perffncacheload
+  $ hg debugrebuildfncache
+  fncache already up to date
   $ hg perffncachewrite
+  $ hg debugrebuildfncache
+  fncache already up to date
 #endif
   $ hg perfheads
   $ hg perfindex