bookmarks: guard against listing bookmarks on unsupported repos

This fixes clones and pulls from statichttprepository repos.

hgext/bookmarks.py

@@ -370,6 +370,11 b' def reposetup(ui, repo):'
     repo.__class__ = bookmark_repo
 
 def listbookmarks(repo):
+    # We may try to list bookmarks on a repo type that does not
+    # support it (e.g., statichttprepository).
+    if not hasattr(repo, '_bookmarks'):
+        return {}
+
     d = {}
     for k, v in repo._bookmarks.iteritems():
         d[k] = hex(v)