new: Improved workaround for mistaken new directories

Currently, notmuch new assumes any directory with a database mtime of
0 is new, but we don't set the mtime until after processing messages
and subdirectories in that directory.  Hence, anything that prevents
the mtime update (such as an interruption or the wall-clock logic
introduced in 8c39e8d6) will cause the next notmuch new to think the
directory is still new.

We work around this by setting the new directory's database mtime to
-1 before scanning anything in the new directory.  This also obviates
the need for the workaround used in 8c39e8d6.