coccinelle: make use of the "type" FREE_AND_NULL() rule

Apply the result of the just-added coccinelle rule. This manually
excludes a few occurrences, mostly things that resulted in many
FREE_AND_NULL() on one line, that'll be manually fixed in a subsequent
change.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -287,8 +287,7 @@ void free_commit_buffer(struct commit *commit)
 {
 	struct commit_buffer *v = buffer_slab_peek(&buffer_slab, commit);
 	if (v) {
-		free(v->buffer);
-		v->buffer = NULL;
+		FREE_AND_NULL(v->buffer);
 		v->size = 0;
 	}
 }