diff: teach diff to display submodule difference with an inline diff

Teach git-diff and friends a new format for displaying the difference
of a submodule. The new format is an inline diff of the contents of the
submodule between the commit range of the update. This allows the user
to see the actual code change caused by a submodule update.

Add tests for the new format and option.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.h

@@ -111,7 +111,8 @@ enum diff_words_type {
 
 enum diff_submodule_format {
 	DIFF_SUBMODULE_SHORT = 0,
-	DIFF_SUBMODULE_LOG
+	DIFF_SUBMODULE_LOG,
+	DIFF_SUBMODULE_INLINE_DIFF
 };
 
 struct diff_options {