diff: add --default-prefix option

You can change the output of prefixes with diff.noprefix and
diff.mnemonicprefix, but there's no easy way to override them from the
command-line. We do have "--no-prefix", but there's no way to get back
to the default prefix. So let's add an option to do that.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 7d73e97..08ab861 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -852,6 +852,11 @@
 --no-prefix::
 	Do not show any source or destination prefix.
 
+--default-prefix::
+	Use the default source and destination prefixes ("a/" and "b/").
+	This is usually the default already, but may be used to override
+	config such as `diff.noprefix`.
+
 --line-prefix=<prefix>::
 	Prepend an additional prefix to every line of output.