remote-hg: try the 'tip' if no checkout present

There's no concept of HEAD in mercurial, but let's try our best to do
something sensible.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>

git-remote-hg

@@ -439,6 +439,8 @@ def list_head(repo, cur):
         # fake bookmark from current branch
         head = cur
         node = repo['.']
+        if not node:
+            node = repo['tip']
         if not node:
             return
         if head == 'default':