midx: add num_large_offsets to write_midx_context

midx: add num_large_offsets to write_midx_context

In an effort to align write_midx_internal() with the chunk-format API,
continue to group necessary data into "struct write_midx_context". This
change collects the "uint32_t num_large_offsets" into the context. With
this new data, write_midx_large_offsets() now matches the
chunk_write_fn type.

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