commit-graph: minimize commit_graph_data_slab access

commit-graph: minimize commit_graph_data_slab access

In an earlier patch, multiple struct acccesses to `graph_pos` and
`generation` were auto-converted to multiple method calls.

Since the values are fixed and commit-slab access costly, we would be
better off with storing the values as a local variable and reusing it.

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