Integrate diffstat with diff

This creates a generic diffstat function in ui-diff, which then is
invoked from cgit_print_diff with the result that both commit and diff-
view gets a diffstat.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

ui-diff.h

@@ -1,6 +1,9 @@
 #ifndef UI_DIFF_H
 #define UI_DIFF_H
 
+extern void cgit_print_diffstat(const unsigned char *old_sha1,
+				const unsigned char *new_sha1);
+
 extern void cgit_print_diff(const char *new_hex, const char *old_hex,
 			    const char *prefix);