Merge branch 'ds/lazy-load-trees'

The code has been taught to use the duplicated information stored
in the commit-graph file to learn the tree object name for a commit
to avoid opening and parsing the commit object when it makes sense
to do so.

* ds/lazy-load-trees:
  coccinelle: avoid wrong transformation suggestions from commit.cocci
  commit-graph: lazy-load trees for commits
  treewide: replace maybe_tree with accessor methods
  commit: create get_commit_tree() method
  treewide: rename tree to maybe_tree

walker.c

@@ -88,7 +88,7 @@ static int process_commit(struct walker *walker, struct commit *commit)
 
 	walker_say(walker, "walk %s\n", oid_to_hex(&commit->object.oid));
 
-	if (process(walker, &commit->tree->object))
+	if (process(walker, &get_commit_tree(commit)->object))
 		return -1;
 
 	for (parents = commit->parents; parents; parents = parents->next) {