Merge branch 'mt/parallel-checkout-part-3'

The final part of "parallel checkout". * mt/parallel-checkout-part-3: ci: run test round with parallel-checkout enabled parallel-checkout: add tests related to .gitattributes t0028: extract encoding helpers to lib-encoding.sh parallel-checkout: add tests related to path collisions parallel-checkout: add tests for basic operations checkout-index: add parallel checkout support builtin/checkout.c: complete parallel checkout support make_transient_cache_entry(): optionally alloc from mem_pool