Merge branch 'dh/repack' (early part)

Merge branch 'dh/repack' (early part)

* 'dh/repack' (early part):
  Ensure git-repack -a -d --max-pack-size=N deletes correct packs
  pack-objects: clarification & option checks for --max-pack-size
  git-repack --max-pack-size: add option parsing to enable feature
  git-repack --max-pack-size: split packs as asked by write_{object,one}()
  git-repack --max-pack-size: write_{object,one}() respect pack limit
  git-repack --max-pack-size: new file statics and code restructuring
  Alter sha1close() 3rd argument to request flush only