make inline is_null_sha1 global

Replace sha1 comparisons to null_sha1 with a global inline (which previously an
unused static inline in builtin-apply.c)

[jc: with a fix from Jonas Fonseca.]

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-diff.c

@@ -68,8 +68,7 @@ static void stuff_change(struct diff_options *opt,
 {
 	struct diff_filespec *one, *two;
 
-	if (memcmp(null_sha1, old_sha1, 20) &&
-	    memcmp(null_sha1, new_sha1, 20) &&
+	if (!is_null_sha1(old_sha1) && !is_null_sha1(new_sha1) &&
 	    !memcmp(old_sha1, new_sha1, 20))
 		return;