Merge branch 'ds/sparse-checkout-clean' into jch

"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:
  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

Author: gitster

Documentation/git-sparse-checkout.adoc

@@ -9,7 +9,7 @@ git-sparse-checkout - Reduce your working tree to a subset of tracked files
 SYNOPSIS
 --------
 [verse]
-'git sparse-checkout' (init | list | set | add | reapply | disable | check-rules) [<options>]
+'git sparse-checkout' (init | list | set | add | reapply | disable | check-rules | clean) [<options>]
 
 
 DESCRIPTION
@@ -111,6 +111,37 @@ flags, with the same meaning as the flags from the `set` command, in order
 to change which sparsity mode you are using without needing to also respecify
 all sparsity paths.
 
+'clean'::
+	Opportunistically remove files outside of the sparse-checkout
+	definition. This command requires cone mode to use recursive
+	directory matches to determine which files should be removed. A
+	file is considered for removal if it is contained within a tracked
+	directory that is outside of the sparse-checkout definition.
++
+Some special cases, such as merge conflicts or modified files outside of
+the sparse-checkout definition could lead to keeping files that would
+otherwise be removed. Resolve conflicts, stage modifications, and use
+`git sparse-checkout reapply` in conjunction with `git sparse-checkout
+clean` to resolve these cases.
++
+This command can be used to be sure the sparse index works efficiently,
+though it does not require enabling the sparse index feature via the
+`index.sparse=true` configuration.
++
+To prevent accidental deletion of worktree files, the `clean` subcommand
+will not delete any files without the `-f` or `--force` option, unless
+the `clean.requireForce` config option is set to `false`.
++
+The `--dry-run` option will list the directories that would be removed
+without deleting them. Running in this mode can be helpful to predict the
+behavior of the clean comand or to determine which kinds of files are left
+in the sparse directories.
++
+The `--verbose` option will list every file within the directories that
+are considered for removal. This option is helpful to determine if those
+files are actually important or perhaps to explain why the directory is
+still present despite the current sparse-checkout.
+
 'disable'::
 	Disable the `core.sparseCheckout` config setting, and restore the
 	working directory to include all files.