configitems: fixup default value of diff config option

configitems: fixup default value of diff config option

It turned out that configbool is used for most of them so the registered
default value was wrong.

--- a/mercurial/configitems.py	Sun Oct 08 21:42:32 2017 +0200
+++ b/mercurial/configitems.py	Fri Oct 13 22:08:55 2017 +0200
@@ -248,34 +248,34 @@
     default=None,
 )
 coreconfigitem('diff', 'nodates',
-    default=None,
+    default=False,
 )
 coreconfigitem('diff', 'showfunc',
-    default=None,
+    default=False,
 )
 coreconfigitem('diff', 'unified',
     default=None,
 )
 coreconfigitem('diff', 'git',
-    default=None,
+    default=False,
 )
 coreconfigitem('diff', 'ignorews',
-    default=None,
+    default=False,
 )
 coreconfigitem('diff', 'ignorewsamount',
-    default=None,
+    default=False,
 )
 coreconfigitem('diff', 'ignoreblanklines',
-    default=None,
+    default=False,
 )
 coreconfigitem('diff', 'ignorewseol',
-    default=None,
+    default=False,
 )
 coreconfigitem('diff', 'nobinary',
-    default=None,
+    default=False,
 )
 coreconfigitem('diff', 'noprefix',
-    default=None,
+    default=False,
 )
 coreconfigitem('email', 'bcc',
     default=None,