Merge branch 'tb/midx-write'

Code clean-up by splitting code responsible for writing midx files
into its own file.

* tb/midx-write:
  midx-write.c: use `--stdin-packs` when repacking
  midx-write.c: check count of packs to repack after grouping
  midx-write.c: factor out common want_included_pack() routine
  midx-write: move writing-related functions from midx.c