Fix bug with updating agenda lines with correct tags.

Fix bug with updating agenda lines with correct tags.

Updating agenda lines did display tags incorrectly, because it used
the list of tags available as a property to remake the list.  However,
that list contains down-cased versions of the tags, because all the
filtering in the agenda does ignore case in tags.

This patch implements a fresh scan for the tags each time a line is
updated, and in this way fixes the problem.  It also reverses a
previous attempt to fix a similar problem.