Merge branch 'ds/sparse-checkout-clean'

"git sparse-checkout" subcommand learned a new "clean" action to
prune otherwise unused working-tree files that are outside the
areas of interest.

* ds/sparse-checkout-clean:
  sparse-index: improve advice message instructions
  t: expand tests around sparse merges and clean
  sparse-index: point users to new 'clean' action
  sparse-checkout: add --verbose option to 'clean'
  dir: add generic "walk all files" helper
  sparse-checkout: match some 'clean' behavior
  sparse-checkout: add basics of 'clean' command
  sparse-checkout: remove use of the_repository

diff --cc Documentation/git-sparse-checkout.adoc
Simple merge

diff --cc dir.c
Simple merge