revset-benchmark: use a generic revset to test `heads(commonancestors())`
This allow to benchmark revset performance in other repositories than just
the mercurial one.
--- a/contrib/all-revsets.txt Mon Jul 16 16:22:43 2018 +0200
+++ b/contrib/all-revsets.txt Mon Jul 16 16:43:35 2018 +0200
@@ -137,4 +137,4 @@
secret()
# test finding common ancestors
-heads(commonancestors(46879 + ae7f))
+heads(commonancestors(last(head(), 2)))