treewide: replace maybe_tree with accessor methods

In anticipation of making trees load lazily, create a Coccinelle
script (contrib/coccinelle/commit.cocci) to ensure that all
references to the 'maybe_tree' member of struct commit are either
mutations or accesses through get_commit_tree() or
get_commit_tree_oid().

Apply the Coccinelle script to create the rest of the patch.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -1064,8 +1064,8 @@ static void make_cover_letter(struct rev_info *rev, int use_stdout,
 
 	diff_setup_done(&opts);
 
-	diff_tree_oid(&origin->maybe_tree->object.oid,
-		      &head->maybe_tree->object.oid,
+	diff_tree_oid(get_commit_tree_oid(origin),
+		      get_commit_tree_oid(head),
 		      "", &opts);
 	diffcore_std(&opts);
 	diff_flush(&opts);