diff.h: move declaration for global in diff.c from cache.h

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.h b/diff.h
index 6c10ce2..5a0c91b 100644
--- a/diff.h
+++ b/diff.h
@@ -701,4 +701,6 @@ void print_stat_summary(FILE *fp, int files,
 			int insertions, int deletions);
 void setup_diff_pager(struct diff_options *);
 
+extern int diff_auto_refresh_index;
+
 #endif /* DIFF_H */