Exception when latest change on referenced ticket is not a comment

[ejucovy]

If you reference a ticket whose latest modification is not a comment, the plugin will fail with an error. To reproduce:

1. Create a ticket
2. Comment on that ticket
3. Change the ticket's priority from major to minor
4. Create a second ticket
5. Comment on the second ticket with a reference to the first ticket

The plugin will fail with a ValueError, because it assumes that the latest changelog entry in t.get_changelog() will always be a comment.

When running against trac trunk, the plugin raises an exception:

File "ve/src/trac/trac/ticket/model.py", line 360, in save_changes
  listener.ticket_changed(self, comment, author, old_values)
File "ve/src/tracbacksplugin/tracbacks/tracbacks.py", line 103, in ticket_changed
  cnum_lastcomment = int(cnum_lastchange[-1])
ValueError: invalid literal for int() with base 10: 'minor'

I haven't tested against earlier versions of Trac to see if it's a new issue. It does seem to be related to #9008 -- and a commenter (comment:1:ticket:9008) posted a traceback there, against trac 0.12.2, which is functionally identical to the traceback I'm seeing.

The following patch fixes the problem by filtering the referenced-ticket's changelog to only include comments:

tracbacks/tracbacks.py

@@ -95 +95 @@
                 # by the TicketChangePlugin to identify the comment being
                 # edited, so we make sure to add it here.
                 cnum_thischange = 1                
-                change_log = t.get_changelog()
+                change_log = [i for i in t.get_changelog() if i[2] == "comment"]
                 if change_log != []:
                     lastchange = change_log[-1]
                     cnum_lastchange = lastchange[3].rsplit('.', 1)

[ejucovy]

(In [12080]) ticket.get_changelog() returns entries that aren't comments (as well as entries that are comments) -- so, if the latest modification on the ticket does not have a comment, tracbacks will cause an internal error by assuming an incorrect data structure. this commit fixes the problem by filtering out all changelog entries that are not comments. (fixes #9743)