commit-graph.c: prevent overflow in `split_graph_merge_strategy()`

commit-graph.c: prevent overflow in `split_graph_merge_strategy()`

In a similar spirit as previous commits, ensure that we don't overflow
when choosing how to split and merge different layers of the
commit-graph.

In particular, avoid a potential overflow between `size_mult` and
`num_commits`, as well as a potential overflow between the number of
commits currently in the merged graph, and the number of commits in the
graph about to be merged.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>