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>

contrib/remote-helpers/git-remote-hg

diff --git a/contrib/remote-helpers/git-remote-hg b/contrib/remote-helpers/git-remote-hg
index bf5e5b4..a9a1e8f 100755
--- a/contrib/remote-helpers/git-remote-hg
+++ b/contrib/remote-helpers/git-remote-hg
@@ -440,6 +440,8 @@
         head = cur
         node = repo['.']
         if not node:
+            node = repo['tip']
+        if not node:
             return
         if head == 'default':
             head = 'master'