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

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

When writing a commit-graph, we use the chunk-format API to write out
each individual chunk of the commit-graph. Each chunk of the
commit-graph is tracked via a call to `add_chunk()`, along with the
expected size of that chunk.

Similar to an earlier commit which handled the identical issue in the
MIDX machinery, guard against overflow when dealing with a commit-graph
with a large number of entries to avoid corrupting the contents of the
commit-graph itself.

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