emacs: tree: use tag-format-tags

emacs: tree: use tag-format-tags

Previously tree did not use tag-format-tags: since tree wants to
distinguish matching messages from non-matching messages it is not a
perfect fit.

However, in preparation for allowing tag-changes to be shown (i.e.,
added or deleted tags to be indicated) it is convenient to make all
places displaying tags call the same routines.

We modify notmuch-tag-format-tags slightly so that it can take and
argument for the default characteristics of the face before the
special tag features are applied.

This also means that things like the star symbol for flagged messages
all work in tree.