commands: check for modheads being None

Python 2 allows the > operator to be used with a None and an
int. Python 3 does not. So we need to ensure the value isn't None
before comparing with >.

Differential Revision: https://phab.mercurial-scm.org/D5702

mercurial/commands.py

@@ -4362,7 +4362,7 b' def postincoming(ui, repo, modheads, opt'
             msg = _("not updating: %s") % stringutil.forcebytestr(inst)
             hint = inst.hint
             raise error.UpdateAbort(msg, hint=hint)
-    if modheads > 1:
+    if modheads is not None and modheads > 1:
         currentbranchheads = len(repo.branchheads())
         if currentbranchheads == modheads:
             ui.status(_("(run 'hg heads' to see heads, 'hg merge' to merge)\n"))