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>

t/t4013/diff.format-patch_--stdout_--numbered_initial..master

@@ -0,0 +1,127 @@
+$ git format-patch --stdout --numbered initial..master
+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 1/3] Second
+
+This is the second commit.
+---
+ dir/sub |    2 ++
+ file0   |    3 +++
+ file2   |    3 ---
+ 3 files changed, 5 insertions(+), 3 deletions(-)
+ delete mode 100644 file2
+
+diff --git a/dir/sub b/dir/sub
+index 35d242b..8422d40 100644
+--- a/dir/sub
++++ b/dir/sub
+@@ -1,2 +1,4 @@
+ A
+ B
++C
++D
+diff --git a/file0 b/file0
+index 01e79c3..b414108 100644
+--- a/file0
++++ b/file0
+@@ -1,3 +1,6 @@
+ 1
+ 2
+ 3
++4
++5
++6
+diff --git a/file2 b/file2
+deleted file mode 100644
+index 01e79c3..0000000
+--- a/file2
++++ /dev/null
+@@ -1,3 +0,0 @@
+-1
+-2
+-3
+-- 
+g-i-t--v-e-r-s-i-o-n
+
+
+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 2/3] Third
+
+---
+ dir/sub |    2 ++
+ file1   |    3 +++
+ 2 files changed, 5 insertions(+), 0 deletions(-)
+ create mode 100644 file1
+
+diff --git a/dir/sub b/dir/sub
+index 8422d40..cead32e 100644
+--- a/dir/sub
++++ b/dir/sub
+@@ -2,3 +2,5 @@ A
+ B
+ C
+ D
++E
++F
+diff --git a/file1 b/file1
+new file mode 100644
+index 0000000..b1e6722
+--- /dev/null
++++ b/file1
+@@ -0,0 +1,3 @@
++A
++B
++C
+-- 
+g-i-t--v-e-r-s-i-o-n
+
+
+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 3/3] Side
+
+---
+ dir/sub |    2 ++
+ file0   |    3 +++
+ file3   |    4 ++++
+ 3 files changed, 9 insertions(+), 0 deletions(-)
+ create mode 100644 file3
+
+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
+
+$