emacs: tree: make tag updates show in the message pane

emacs: tree: make tag updates show in the message pane

This makes tag changes appear in the message pane as well as in the
tree window.

Note that the message pane is reloaded each time a message is viewed
so the tags shown in the message pane can still be different from
those in the tree window. Usually this will just be that the tag
change is still shown as a change (strikethough underline etc) in the
tree window, and are shown after the change in the message
pane. However, if something else updates the database then the tags
shown can be genuinely different.