perf: add perfmoonwalk command to walk the changelog backwards

This lets us test the effectiveness (or ineffectiveness) of the revlog chunk
cache when walking revlogs backwards.

contrib/perf.py

@@ -243,6 +243,18 b' def perflog(ui, repo, **opts):'
                                copies=opts.get('rename')))
     ui.popbuffer()
 
+@command('perfmoonwalk')
+def perfmoonwalk(ui, repo):
+    """benchmark walking the changelog backwards
+
+    This also loads the changelog data for each revision in the changelog.
+    """
+    def moonwalk():
+        for i in xrange(len(repo), -1, -1):
+            ctx = repo[i]
+            ctx.branch() # read changelog data (in addition to the index)
+    timer(moonwalk)
+
 @command('perftemplating')
 def perftemplating(ui, repo):
     ui.pushbuffer()