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: Tim McCormack <cortex@brainonfire.net> Suggested-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>