commit-graph: don't call write_graph_chunk_extra_edges() unnecessarily

The optional 'Extra Edge List' chunk of the commit graph file stores parent information for commits with more than two parents. Since the chunk is optional, write_commit_graph() looks through all commits to find those with more than two parents, and then writes the commit graph file header accordingly, i.e. if there are no such commits, then there won't be a 'Extra Edge List' chunk written, only the three mandatory chunks. However, when it later comes to writing actual chunk data, write_commit_graph() unconditionally invokes write_graph_chunk_extra_edges(), even when it was decided earlier that that chunk won't be written. Strictly speaking there is no bug here, because write_graph_chunk_extra_edges() won't write anything if it doesn't find any commits with more than two parents, but then it unnecessarily and in vain looks through all commits once again in search for such commits. Don't call write_graph_chunk_extra_edges() when that chunk won't be written to spare an unnecessary iteration over all commits. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>