perf: move revlog construction and length calculation out of benchmark

We don't need to measure the time it takes to open the revlog or
calculate its length.

This is more consistent with what other perf* functions do.

While I was here, I also renamed the revlog variable from "r" to
"rl" - again in the name of consistency.

contrib/perf.py

@@ -850,14 +850,14 b' def perfrevlog(ui, repo, file_=None, sta'
 
     The start revision can be defined via ``-s/--startrev``.
     """
-    _len = getlen(ui)
+    rl = cmdutil.openrevlog(repo, 'perfrevlog', file_, opts)
+    rllen = getlen(ui)(rl)
 
     def d():
-        r = cmdutil.openrevlog(repo, 'perfrevlog', file_, opts)
-        r.clearcaches()
+        rl.clearcaches()
 
         beginrev = startrev
-        endrev = _len(r)
+        endrev = rllen
         dist = opts['dist']
 
         if reverse:
@@ -865,7 +865,7 b' def perfrevlog(ui, repo, file_=None, sta'
             dist = -1 * dist
 
         for x in xrange(beginrev, endrev, dist):
-            r.revision(x)
+            rl.revision(x)
 
     timer, fm = gettimer(ui, opts)
     timer(d)