The existing --attach option did not create a true "attachment"
but multipart/mixed with Content-Disposition: inline.  It should
have been with Content-Disposition: attachment.

Introduce --inline to add multipart/mixed that is inlined, and
make --attach to create an attachement.
Signed-off-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <junkio@cox.net>