Merge branch 'tb/enable-cruft-packs-by-default'

Merge branch 'tb/enable-cruft-packs-by-default'

When "gc" needs to retain unreachable objects, packing them into
cruft packs (instead of exploding them into loose object files) has
been offered as a more efficient option for some time.  Now the use
of cruft packs has been made the default and no longer considered
an experimental feature.

* tb/enable-cruft-packs-by-default:
  repository.h: drop unused `gc_cruft_packs`
  builtin/gc.c: make `gc.cruftPacks` enabled by default
  t/t9300-fast-import.sh: prepare for `gc --cruft` by default
  t/t6500-gc.sh: add additional test cases
  t/t6500-gc.sh: refactor cruft pack tests
  t/t6501-freshen-objects.sh: prepare for `gc --cruft` by default
  t/t5304-prune.sh: prepare for `gc --cruft` by default
  builtin/gc.c: ignore cruft packs with `--keep-largest-pack`
  builtin/repack.c: fix incorrect reference to '-C'
  pack-write.c: plug a leak in stage_tmp_packfiles()