sparse cleanup

Fix various things that sparse complains about:
 - use NULL instead of 0
 - make sure we declare everything properly, or mark it static
 - use proper function declarations ("fn(void)" instead of "fn()")

Sparse is always right.

diff-files.c

@@ -86,7 +86,7 @@ int main(int argc, char **argv)
 
 	diff_setup(detect_rename, diff_score_opt, reverse_diff,
 		   (generate_patch ? -1 : line_termination),
-		   0, 0);
+		   NULL, 0);
 
 	for (i = 0; i < entries; i++) {
 		struct stat st;