revset: add test demonstrating a bug with commonancestor()

Specifically, 9 is clearly not in "::8 and ::9".

Differential Revision: https://phab.mercurial-scm.org/D4737

tests/test-revset.t

@@ -1063,6 +1063,14 b' test common ancestors'
   8
   9
 
+  $ hg log -T '{rev}\n' -r 'commonancestors(8 + 9)'
+  0
+  1
+  2
+  4
+  8
+  9
+
 test ancestor variants of empty revision
 
   $ log 'ancestor(none())'