Emit `tag::history::update` from tag_view_only_byindex

This triggers awful.tag to update its tag history.

Fixes: https://awesome.naquadah.org/bugs/index.php?do=details&task_id=1217

As discussed on IRC, the C code should not really emit this signal
(which is created by awful.tag).

In case the signal is unknown, a warning will be given by awesome (to
stderr).