Roundup form-processing allows you to create a new item by giving it a
negative ID.  For example:

       <textarea name="msg-2@content" rows="6" cols="81"
                 wrap="soft" class="sourcerous"></textarea>
       <input type="hidden" name="@link@comments" value="msg-2" /> 
       <input type="submit" value="Submit Changes" />

The "msg-2@content" bit says that the value of that <textarea> is the
content of a new msg.  (The first new msg is msg-1, the second is
msg-2, etc.)

Except that Roundup's form-processing logic incorrectly looks for "-1"
in some places.  It uses "startswith('-1')" in most places, e.g.:

                    elif nodeid and not nodeid.startswith('-'):
                        existing = cl.get(nodeid, propname, [])

but not in the place I found below.

This used to work with 1.3.3 because the EditItemAction class had some
compensating logic.  That went away, but the form-parsing issue was
not fixed.

Applied with changes (handle id is None and fix incorrect unit test)