commit-graph: create write_commit_graph_context

The write_commit_graph() method is too large and complex. To simplify
it, we should extract several helper functions. However, we will risk
repeating a lot of declarations related to progress incidators and
object id or commit lists.

Create a new write_commit_graph_context struct that contains the
core data structures used in this process. Replace the other local
variables with the values inside the context object. Following this
change, we will start to lift code segments wholesale out of the
write_commit_graph() method and into helper functions.

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