revlog: clarify which version use the older API in perf

This work is part of a refactoring to unify the revlog index and the nodemap.
This unification prepare the use of a persistent nodemap.

Differential Revision: https://phab.mercurial-scm.org/D7315

contrib/perf.py

@@ -820,6 +820,7 b' def clearcaches(cl):'
     if util.safehasattr(cl, b'clearcaches'):
         cl.clearcaches()
     elif util.safehasattr(cl, b'_nodecache'):
+        # <= hg-5.2
         from mercurial.node import nullid, nullrev
 
         cl._nodecache = {nullid: nullrev}