Merge branch 'ep/maint-equals-null-cocci'

Introduce and apply coccinelle rule to discourage an explicit
comparison between a pointer and NULL, and applies the clean-up to
the maintenance track.

* ep/maint-equals-null-cocci:
  tree-wide: apply equals-null.cocci
  tree-wide: apply equals-null.cocci
  contrib/coccinnelle: add equals-null.cocci

shallow.c

@@ -560,7 +560,7 @@ static void paint_down(struct paint_info *info, const struct object_id *oid,
 		else
 			c->object.flags |= SEEN;
 
-		if (*refs == NULL)
+		if (!*refs)
 			*refs = bitmap;
 		else {
 			memcpy(tmp, *refs, bitmap_size);