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>
diff --git a/log-tree.c b/log-tree.c
index 6ce239d..7b3ad7d 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -205,10 +205,12 @@
 				 "Content-Type: text/x-patch;\n"
 				 " name=\"%s.diff\"\n"
 				 "Content-Transfer-Encoding: 8bit\n"
-				 "Content-Disposition: inline;\n"
+				 "Content-Disposition: %s;\n"
 				 " filename=\"%s.diff\"\n\n",
 				 mime_boundary_leader, opt->mime_boundary,
-				 sha1, sha1);
+				 sha1,
+				 opt->no_inline ? "attachment" : "inline",
+				 sha1);
 			opt->diffopt.stat_sep = buffer;
 		}
 	} else if (opt->commit_format != CMIT_FMT_USERFORMAT) {