Merge branch 'ds/default-pack-use-sparse-to-true'

The 'pack.useSparse' configuration variable now defaults to 'true',
enabling an optimization that has been experimental since Git 2.21.

* ds/default-pack-use-sparse-to-true:
  pack-objects: flip the use of GIT_TEST_PACK_SPARSE
  config: set pack.useSparse=true by default

diff --cc builtin/pack-objects.c
Simple merge

diff --cc t/README
Simple merge