commit-graph: pass graphs that are to be merged as parameter

commit-graph: pass graphs that are to be merged as parameter

When determining whether or not we want to merge a commit graph chain we
retrieve the graph that is to be merged via the context's repository.
With an upcoming change though it will become a bit more complex to
figure out the commit graph, which would lead to code duplication.

Prepare for this change by passing the graph that is to be merged as a
parameter.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>