perf: call clearcaches() in perfmanifest

The old code only partially cleared the caches. Now that we have a
comprehensive method for wiping all caches, let's call it.

This appears to introduce a marginal regression in `hg perfmanifest`
on mozilla-central. This is good because the new result is more
accurate since caches aren't being used.

contrib/perf.py

@@ -273,8 +273,7 b' def perfmanifest(ui, repo, rev, **opts):'
     ctx = scmutil.revsingle(repo, rev, rev)
     t = ctx.manifestnode()
     def d():
-        repo.manifest._mancache.clear()
-        repo.manifest._cache = None
+        repo.manifest.clearcaches()
         repo.manifest.read(t)
     timer(d)
     fm.end()