midx: extract bitmap write setup

midx: extract bitmap write setup

The write_midx_bitmap() method is a long method that does a lot of
steps. It requires the write_midx_context struct for use in
prepare_midx_packing_data() and find_commits_for_midx_bitmap(), but
after that only needs the pack_order array.

This is a messy, but completely non-functional refactoring. The code is
only being moved around to reduce visibility of the write_midx_context
during the longest part of computing reachability bitmaps.

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