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>

t/t4013/diff.format-patch_--inline_--stdout_initial..side

@@ -0,0 +1,62 @@
+$ git format-patch --inline --stdout initial..side
+From c7a2ab9e8eac7b117442a607d5a9b3950ae34d5a Mon Sep 17 00:00:00 2001
+From: A U Thor <author@example.com>
+Date: Mon, 26 Jun 2006 00:03:00 +0000
+Subject: [PATCH] Side
+MIME-Version: 1.0
+Content-Type: multipart/mixed;
+ boundary="------------g-i-t--v-e-r-s-i-o-n"
+
+This is a multi-part message in MIME format.
+--------------g-i-t--v-e-r-s-i-o-n
+Content-Type: text/plain; charset=UTF-8; format=fixed
+Content-Transfer-Encoding: 8bit
+
+---
+ dir/sub |    2 ++
+ file0   |    3 +++
+ file3   |    4 ++++
+ 3 files changed, 9 insertions(+), 0 deletions(-)
+ create mode 100644 file3
+--------------g-i-t--v-e-r-s-i-o-n
+Content-Type: text/x-patch;
+ name="c7a2ab9e8eac7b117442a607d5a9b3950ae34d5a.diff"
+Content-Transfer-Encoding: 8bit
+Content-Disposition: inline;
+ filename="c7a2ab9e8eac7b117442a607d5a9b3950ae34d5a.diff"
+
+diff --git a/dir/sub b/dir/sub
+index 35d242b..7289e35 100644
+--- a/dir/sub
++++ b/dir/sub
+@@ -1,2 +1,4 @@
+ A
+ B
++1
++2
+diff --git a/file0 b/file0
+index 01e79c3..f4615da 100644
+--- a/file0
++++ b/file0
+@@ -1,3 +1,6 @@
+ 1
+ 2
+ 3
++A
++B
++C
+diff --git a/file3 b/file3
+new file mode 100644
+index 0000000..7289e35
+--- /dev/null
++++ b/file3
+@@ -0,0 +1,4 @@
++A
++B
++1
++2
+
+--------------g-i-t--v-e-r-s-i-o-n--
+
+
+$