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>
diff --git a/merge-recursive.c b/merge-recursive.c
index 0f75d75..8de6e36 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -2154,8 +2154,8 @@ int merge_recursive(struct merge_options *o,
 		read_cache();
 
 	o->ancestor = "merged common ancestors";
-	clean = merge_trees(o, h1->maybe_tree, h2->maybe_tree,
-			    merged_common_ancestors->maybe_tree,
+	clean = merge_trees(o, get_commit_tree(h1), get_commit_tree(h2),
+			    get_commit_tree(merged_common_ancestors),
 			    &mrtree);
 	if (clean < 0) {
 		flush_output(o);