revsetbenchmark: add some simpler revset for heads and roots

This revset might leverage compiled code in the future so lets start to track
them.

contrib/base-revsets.txt

@@ -47,3 +47,6 b' parents(20000)'
 # The one below is used by rebase
 (children(ancestor(tip~5, tip)) and ::(tip~5))::
 heads(commonancestors(last(head(), 2)))
+heads(-10000:-1)
+roots(-10000:-1)
+only(max(head()), min(head()))