Merge branch 'ds/midx-repack-to-batch-size'

The "--batch-size" option of "git multi-pack-index repack" command
is now used to specify that very small packfiles are collected into
one until the total size roughly exceeds it.

* ds/midx-repack-to-batch-size:
  multi-pack-index: repack batches below --batch-size

diff --cc midx.c
Simple merge

diff --cc t/t5319-multi-pack-index.sh
Simple merge