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