notmuch new: Fix to detect deletions of names at the end of the list.

Previously we only scanned the list of filenames in the filesystem and
detected a deletion whenever that scan skipped a name that existed in
the database. That much was fine, but we *also* need to continue
walking the list of names from the database when the filesystem list
is exhausted.

Without this, removing the last file or directory within any
particular directory would go undetected.