Properly detect missing contexts

This can happen when there's a synchronization issue between marks-git
and marks-hg; a key is missing in marks-hg, and when we receive a reset
command the value of ctx basically comes from None.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>

git-remote-hg

@@ -914,6 +914,11 @@ def checkheads_bmark(repo, ref, ctx):
 
     ctx_old = bmarks[bmark]
     ctx_new = ctx
+
+    if not ctx.rev():
+        print "error %s unknown" % ref
+        return False
+
     if not repo.changelog.descendant(ctx_old.rev(), ctx_new.rev()):
         if force_push:
             print "ok %s forced update" % ref