Tag contents, once read, are forever cached in memory.
This makes gitk unable to notice when tag contents change.

Allow users to cause a reload of the tag contents by using
the "File->Reread references" action.
Reported-by: NTim McCormack <cortex@brainonfire.net>
Suggested-by: NJunio C Hamano <gitster@pobox.com>
Signed-off-by: NDavid Aguilar <davvid@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>