revset: add larger test for heads(ancestors(…))

It is important to not regress on this benchmark so we move it into the "base"
file. And we add another benchmark with more than two revisions.

contrib/all-revsets.txt

@@ -138,3 +138,4 b' secret()'
 
 # test finding common ancestors
 heads(commonancestors(last(head(), 2)))
+heads(commonancestors(head()))

contrib/base-revsets.txt

@@ -46,3 +46,4 b' parents(20000)'
 (20000::) - (20000)
 # The one below is used by rebase
 (children(ancestor(tip~5, tip)) and ::(tip~5))::
+heads(commonancestors(last(head(), 2)))