Merge branch 'ds/sparse-lstat-caching'

The code to deal with modified paths that are out-of-cone in a
sparsely checked out working tree has been optimized.

* ds/sparse-lstat-caching:
  sparse-index: improve lstat caching of sparse paths
  sparse-index: count lstat() calls
  sparse-index: use strbuf in path_found()
  sparse-index: refactor path_found()
  sparse-checkout: refactor skip worktree retry logic