revsetbenchmark: add entry for ::rev::

Revsets of the form ::rev:: were identified as the source behind the
regressions in issue 4201. Ensure we have explicit coverage of that
revset.

contrib/revsetbenchmarks.txt

@@ -1,14 +1,15 b''
 all()
 draft()
 ::tip
 draft() and ::tip
 0::tip
 roots(0::tip)
 author(lmoscovicz)
 author(mpm)
 author(lmoscovicz) or author(mpm)
 tip:0
 max(tip:0)
 min(0:tip)
 0::
 min(0::)
+::p1(p1(tip))::