Merge branch 'tb/multi-cruft-pack'

Use of --max-pack-size to allow multiple packfiles to be created is
now supported even when we are sending unreachable objects to cruft
packs.

* tb/multi-cruft-pack:
  Documentation/gitformat-pack.txt: drop mixed version section
  Documentation/gitformat-pack.txt: remove multi-cruft packs alternative
  builtin/pack-objects.c: support `--max-pack-size` with `--cruft`
  builtin/pack-objects.c: remove unnecessary strbuf_reset()