emacs: Move search-target logic to `notmuch-search-show-result'

This is a simpler place to do this, since we can avoid any point
motion and hence any save-excursions in
`notmuch-search-process-filter', which in turn lets us put all of the
search-target logic outside of any save-excursions.

`notmuch-search-show-{result,error}' are now responsible for their own
point motion.

`notmuch-search-process-filter' could use some reindentation after
this, but we're about to rewrite it entirely, so we won't bother.