py3: add a missing b'' prefix in contrib/perf.py

# skip-blame because just b'' prefixes

This fixes test-contrib-perf.t on Python 3 which started failing.

Differential Revision: https://phab.mercurial-scm.org/D5421

contrib/perf.py

@@ -1014,7 +1014,7 b' def perfignore(ui, repo, **opts):'
     fm.end()
 
 @command(b'perfindex', [
-            (b'', b'rev', '', b'revision to be looked up (default tip)'),
+            (b'', b'rev', b'', b'revision to be looked up (default tip)'),
          ] + formatteropts)
 def perfindex(ui, repo, **opts):
     import mercurial.revlog