format-patch: autonumber by default

format-patch is most commonly used for multiple patches at once when
sending a patchset, in which case we want to number the patches; on
the other hand, single patches are not usually expected to be
numbered.

In other words, the typical behavior expected from format-patch is the
one obtained by enabling autonumber, so we set it to be the default.

Users that want to disable numbering for a particular patchset can do
so with the existing -N command-line switch.  Users that want to
change the default behavior can use the format.numbering config key.

Signed-off-by: Brian Gernhardt <benji@silverinsanity.com>
Test-updates-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t4013/diff.format-patch_--attach_--stdout_initial..master^ b/t/t4013/diff.format-patch_--attach_--stdout_initial..master^
index d7490a9..2c71d20 100644
--- a/t/t4013/diff.format-patch_--attach_--stdout_initial..master^
+++ b/t/t4013/diff.format-patch_--attach_--stdout_initial..master^
@@ -2,7 +2,7 @@
 From 1bde4ae5f36c8d9abe3a0fce0c6aab3c4a12fe44 Mon Sep 17 00:00:00 2001
 From: A U Thor <author@example.com>
 Date: Mon, 26 Jun 2006 00:01:00 +0000
-Subject: [PATCH] Second
+Subject: [PATCH 1/2] Second
 MIME-Version: 1.0
 Content-Type: multipart/mixed; boundary="------------g-i-t--v-e-r-s-i-o-n"
 
@@ -63,7 +63,7 @@
 From 9a6d4949b6b76956d9d5e26f2791ec2ceff5fdc0 Mon Sep 17 00:00:00 2001
 From: A U Thor <author@example.com>
 Date: Mon, 26 Jun 2006 00:02:00 +0000
-Subject: [PATCH] Third
+Subject: [PATCH 2/2] Third
 MIME-Version: 1.0
 Content-Type: multipart/mixed; boundary="------------g-i-t--v-e-r-s-i-o-n"