add David Turner's Two Sigma address

Signed-off-by: David Turner <novalis@novalis.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.mailmap

@@ -48,6 +48,7 @@ David Kågedal <davidk@lysator.liu.se>
 David Reiss <dreiss@facebook.com> <dreiss@dreiss-vmware.(none)>
 David S. Miller <davem@davemloft.net>
 David Turner <novalis@novalis.org> <dturner@twopensource.com>
+David Turner <novalis@novalis.org> <dturner@twosigma.com>
 Deskin Miller <deskinm@umich.edu>
 Dirk Süsserott <newsletter@dirk.my1.cc>
 Eric Blake <eblake@redhat.com> <ebb9@byu.net>