commit-graph: anonymize data in chunk_write_fn

commit-graph: anonymize data in chunk_write_fn

In preparation for creating an API around file formats using chunks and
tables of contents, prepare the commit-graph write code to use
prototypes that will match this new API.

Specifically, convert chunk_write_fn to take a "void *data" parameter
instead of the commit-graph-specific "struct write_commit_graph_context"
pointer.

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