format-patch: Add format.subjectprefix config option

This change lets you use the format.subjectprefix config option to override the
default subject prefix.

Signed-off-by: Adam Roben <aroben@apple.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4013/diff.config_format.subjectprefix_DIFFERENT_PREFIX

@@ -0,0 +1,2 @@
+$ git config format.subjectprefix DIFFERENT_PREFIX
+$

t/t4013/diff.format-patch_--inline_--stdout_initial..master^^

@@ -0,0 +1,60 @@
+$ git format-patch --inline --stdout 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: [DIFFERENT_PREFIX] Second
+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
+
+
+This is the second commit.
+---
+ dir/sub |    2 ++
+ file0   |    3 +++
+ file2   |    3 ---
+ 3 files changed, 5 insertions(+), 3 deletions(-)
+ delete mode 100644 file2
+--------------g-i-t--v-e-r-s-i-o-n
+Content-Type: text/x-patch; name="1bde4ae5f36c8d9abe3a0fce0c6aab3c4a12fe44.diff"
+Content-Transfer-Encoding: 8bit
+Content-Disposition: inline; filename="1bde4ae5f36c8d9abe3a0fce0c6aab3c4a12fe44.diff"
+
+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--
+
+
+$