flag synchronization moves messages from new to cur when not necessary

[Jesse Luehrs]

If a message in new has a trailing ":2," in the filename, notmuch will
move it to cur whenever tags are modified, even if nothing has changed
with tags that are supposed to be synchronized. While it's true that
having messages in new with extra info attached violates the maildir
spec, it does happen with some clients (I ran into it with offlineimap,
and filed a bug https://github.com/OfflineIMAP/offlineimap/issues/80
here, but not sure if or when it will be addressed).

On IRC, it was suggested that it wouldn't really harm anything to
consider messages with a trailing ":2," (with no other flags) in new as
being equivalent to the info not existing at all. Would this be
a possible solution?

-doy