commit-graph: unify the signatures of all write_graph_chunk_*() functions

commit-graph: unify the signatures of all write_graph_chunk_*() functions

Update the write_graph_chunk_*() helper functions to have the same
signature:

  - Return an int error code from all these functions.
    write_graph_chunk_base() already has an int error code, now the
    others will have one, too, but since they don't indicate any
    error, they will always return 0.

  - Drop the hash size parameter of write_graph_chunk_oids() and
    write_graph_chunk_data(); its value can be read directly from
    'the_hash_algo' inside these functions as well.

This opens up the possibility for further cleanups and foolproofing in
the following two patches.

Helped-by: René Scharfe <l.s.r@web.de>
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>