perfmanifest: fix cache invalidation

The manifest cache is now stored in _mancache. The revlog cache is still stored
in _cache.

contrib/perf.py

@@ -175,9 +175,9 b' def perfpathcopies(ui, repo, rev1, rev2)'
 def perfmanifest(ui, repo):
     def d():
         t = repo.manifest.tip()
+        repo.manifest._mancache.clear()
+        repo.manifest._cache = None
         repo.manifest.read(t)
-        repo.manifest.mapcache = None
-        repo.manifest._cache = None
     timer(d)
 
 @command('perfchangeset')