bookmarks: always write undo file

Always write the undo file. Otherwise, rollback will not work for
the initial bookmark as undo.bookmarks doesn't exists. In this case
undo.bookmarks needs already be empty.

hgext/bookmarks.py

@@ -48,9 +48,8 b' def write(repo):'
     try:
         bms = repo.opener('bookmarks').read()
     except IOError:
-        bms = None
-    if bms is not None:
-        repo.opener('undo.bookmarks', 'w').write(bms)
+        bms = ''
+    repo.opener('undo.bookmarks', 'w').write(bms)
 
     if repo._bookmarkcurrent not in refs:
         setcurrent(repo, None)