lib: Treat messages in new/ as maildir messages with no flags set

lib: Treat messages in new/ as maildir messages with no flags set

Previously, notmuch new only synchronized maildir flags to tags for
files with a maildir "info" part.  Since messages in new/ don't have
an info part, notmuch would ignore them for flag-to-tag
synchronization.

This patch makes notmuch consider messages in new/ to be legitimate
maildir messages that simply have no maildir flags set.  The most
visible effect of this is that such messages now automatically get the
unread tag.