revset: add tests of heads(commonancestors(..))

The second-to-last one shows the same bug as commonancestors(..): the
result should be 8, not 9.

Differential Revision: https://phab.mercurial-scm.org/D4741

tests/test-revset.t

@@ -1070,11 +1070,24 test common ancestors
   4
   8
 
+test the specialized implementation of heads(commonancestors(..))
+(2 gcas is tested in test-merge-criss-cross.t)
+
+  $ hg log -T '{rev}\n' -r 'heads(commonancestors(7 + 9))'
+  4
+  $ hg log -T '{rev}\n' -r 'heads(commonancestors(heads(all())))'
+  4
+  $ hg log -T '{rev}\n' -r 'heads(commonancestors(9))'
+  9
+  $ hg log -T '{rev}\n' -r 'heads(commonancestors(8 + 9))'
+  9
+
 test ancestor variants of empty revision
 
   $ log 'ancestor(none())'
   $ log 'ancestors(none())'
   $ log 'commonancestors(none())'
+  $ log 'heads(commonancestors(none()))'
 
 test ancestors with depth limit