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>

contrib/coccinelle/commit.cocci

@@ -0,0 +1,30 @@
+@@
+expression c;
+@@
+- &c->maybe_tree->object.oid
++ get_commit_tree_oid(c)
+
+@@
+expression c;
+@@
+- c->maybe_tree->object.oid.hash
++ get_commit_tree_oid(c)->hash
+
+@@
+expression c;
+@@
+- c->maybe_tree
++ get_commit_tree(c)
+
+@@
+expression c;
+expression s;
+@@
+- get_commit_tree(c) = s
++ c->maybe_tree = s
+
+@@
+expression c;
+@@
+- return get_commit_tree(c);
++ return c->maybe_tree;