format-patch: add --inline option and make --attach a true attachment

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: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

revision.h

@@ -74,6 +74,7 @@ struct rev_info {
 	const char	*add_signoff;
 	const char	*extra_headers;
 	const char	*log_reencode;
+	int		no_inline;
 
 	/* Filter by commit log message */
 	struct grep_opt	*grep_filter;