revision: add missing include

Otherwise we might not have 'struct diff_options'.

[jc: needs a matching follow-up patch to remove inclusion of diff.h
from *.c files that do not themselves use anything from diff.h]

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.h

@@ -5,6 +5,7 @@
 #include "grep.h"
 #include "notes.h"
 #include "commit.h"
+#include "diff.h"
 
 #define SEEN		(1u<<0)
 #define UNINTERESTING   (1u<<1)