Merge branch 'hf/unpack-trees-quadratic-scan'

Merge branch 'hf/unpack-trees-quadratic-scan'

The cache-scanning loop in 'next_cache_entry()' has been optimized
to avoid rescanning already-unpacked index entries, preventing a
quadratic performance slow-down when diffing the working tree
against a commit with a pathspec matching early index entries.

* hf/unpack-trees-quadratic-scan:
  unpack-trees: avoid quadratic index scan in next_cache_entry()