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