[PATCH 1 of 2] revset-benchmark: use a generic revset to test `heads(commonancestors())`

[Boris Feld]

# HG changeset patch
# User Boris Feld <boris.feld at octobus.net>
# Date 1531752215 -7200
#      Mon Jul 16 16:43:35 2018 +0200
# Node ID d401e0d99b2c8e18bda7e007c2cd7599afbb365c
# Parent  3c569172848d329a81b1420d7cd8a5549b48f25f
# EXP-Topic revset-bench
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r d401e0d99b2c
revset-benchmark: use a generic revset to test `heads(commonancestors())`

This allow to benchmark revset performance in other repositories than just
the mercurial one.

diff --git a/contrib/all-revsets.txt b/contrib/all-revsets.txt
--- a/contrib/all-revsets.txt
+++ b/contrib/all-revsets.txt
@@ -137,4 +137,4 @@ draft()
 secret()
 
 # test finding common ancestors
-heads(commonancestors(46879 + ae7f))
+heads(commonancestors(last(head(), 2)))