gitk does not reload tag messages

If a tag that gitk knows about is deleted and recreated with a
different message, gitk still shows the old message after any
combination of refresh, reload, and reread refs.

git-gui version 0.13.0.8.g8f85

Reproduce:
1. git tag -a test -m "foo" HEAD
2. Open gitk, see that correct message ("foo") is present for tag "test"
3. git tag -d test
4. Reload/refresh gitk [optional, it won't make a difference]
5. git tag -a test -m "bar" HEAD
6. Reload/refresh gitk
7. See that the message on tag "test" is still "foo"
8. $ git show test # shows correct message "bar"

Apologies if this is an old version or a known bug; I could not find an
issue tracker to search, although I did check the gmane archives.

 - Tim McCormack