new directory will result in notmuch creating new timestamp documents
and leaving stale ones behind.)
+Ensure that "notmuch new" is sane if its first, giant indexing session
+gets interrupted, (that is, ensure that any results indexed so far are
+flushed).
+
Fix notmuch.c to use a DIR prefix for directory timestamps, (the idea
being that it can then add other non-directory timestamps such as for
noting how far back in the past mail has been indexed, and whether it