commit-graph: extract copy_oids_to_commits()

commit-graph: extract copy_oids_to_commits()

The write_commit_graph() method is too complex, so we are
extracting helper functions one by one.

Extract copy_oids_to_commits(), which fills the commits list
with the distinct commits from the oids list. During this loop,
it also counts the number of "extra" edges from octopus merges.

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