commit: move members graph_pos, generation to a slab

We remove members `graph_pos` and `generation` from the struct commit.
The default assignments in init_commit_node() are no longer valid,
which is fine as the slab helpers return appropriate default values and
the assignments are removed.

We will replace existing use of commit->generation and commit->graph_pos
by commit_graph_data_slab helpers using
`contrib/coccinelle/commit.cocci'.

Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -36,8 +36,6 @@ struct commit {
 	 * or get_commit_tree_oid().
 	 */
 	struct tree *maybe_tree;
-	uint32_t graph_pos;
-	uint32_t generation;
 	unsigned int index;
 };