sparse-checkout: add --verbose option to 'clean'

sparse-checkout: add --verbose option to 'clean'

The 'git sparse-checkout clean' subcommand is focused on directories,
deleting any tracked sparse directories to clean up the worktree and
make the sparse index feature work optimally.

However, this directory-focused approach can leave users wondering why
those directories exist at all. In my experience, these files are left
over due to ignore or exclude patterns, Windows file handles, or
possibly merge conflict resolutions.

Add a new '--verbose' option for users to see all the files that are
being deleted (with '--force') or would be deleted (with '--dry-run').

Based on usage, users may request further context on this list of files for
states such as tracked/untracked, unstaged/staged/conflicted, etc.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>