thread.cc: Avoid empty thread names if possible.

thread.cc: Avoid empty thread names if possible.

Currently the thread is named based on either the oldest or newest
matching message (depending on the search order). If this message has
an empty subject, though, the thread will show up with an empty
subject in the search results. (See the thread starting with
`id:1412371140-21051-1-git-send-email-david@tethera.net` for an
example.)

This changes the behavior so it will use a non-empty name for the
thread if possible. We name threads based on (a) non-empty matches for
the query, and (b) the search order. If the search order is
oldest-first (as in the default inbox) it chooses the oldest matching
non-empty message as the subject. If the search order is newest-first
it chooses the newest one.