revset: fix commonancestor test so it demonstrates correct behavior

The problem is that hg log -r 'head()' is every changeset in the
repository, because in this test repository, every changeset has a
different branch. The author probably assumed all commits were on the
default branch, and that they were getting topological heads, 7 and 9.

As a result, this test was showing that the common ancestors of
0:9 are 0+1+2+4, which is not correct (next commit will test this).

Differential Revision: https://phab.mercurial-scm.org/D4736

tests/test-revset.t

@@ -1049,7 +1049,7 b' test common ancestors'
   2
   4
 
-  $ hg log -T '{rev}\n' -r 'commonancestors(head())'
+  $ hg log -T '{rev}\n' -r 'commonancestors(heads(all()))'
   0
   1
   2