revset: special case commonancestors(none()) to be empty set

This matches the behavior of ancestor(none()).

From an implementation perspective, ancestor() and commonancestors() are
intersection, and ancestors() is union, so it would make some sense that
commonancestors(none()) returned all revisions. However, ancestor(none())
isn't implemented as such, which breaks ancestor(x) == max(commonancestors(x)).

From a user perspective, ancestors of nothing is nothing whichever type
of operation the ancestor predicate does.

mercurial/revset.py

@@ -632,7 +632,10 b' def commonancestors(repo, subset, x):'
 
     """
     # only wants the heads of the set passed in
-    for r in heads(repo, fullreposet(repo), x, anyorder):
+    h = heads(repo, fullreposet(repo), x, anyorder)
+    if not h:
+        return baseset()
+    for r in h:
         subset &= dagop.revancestors(repo, baseset([r]))
 
     return subset

tests/test-revset.t

@@ -1063,6 +1063,12 b' test common ancestors'
   8
   9
 
+test ancestor variants of empty revision
+
+  $ log 'ancestor(none())'
+  $ log 'ancestors(none())'
+  $ log 'commonancestors(none())'
+
 test ancestors with depth limit
 
  (depth=0 selects the node itself)