bookmarks: guard against listing bookmarks on unsupported repos stable

bookmarks: guard against listing bookmarks on unsupported repos

This fixes clones and pulls from statichttprepository repos.

--- a/hgext/bookmarks.py	Sat Aug 21 10:44:57 2010 -0500
+++ b/hgext/bookmarks.py	Mon Aug 23 22:16:56 2010 +0200
@@ -370,6 +370,11 @@
     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)