builtin/sparse-checkout: remove NEED_WORK_TREE flag

builtin/sparse-checkout: remove NEED_WORK_TREE flag

In preparation for adding a sub-command to 'sparse-checkout' that can be
run in a bare repository, remove the 'NEED_WORK_TREE' flag from its
entry in the 'commands' array of 'git.c'.

To avoid that this changes any behaviour, add calls to
'setup_work_tree()' to all of the 'sparse-checkout' sub-commands and add
tests that verify that 'sparse-checkout <cmd>' still fail with a clear
error message telling the user that the command needs a work tree.

Signed-off-by: William Sprent <williams@unity3d.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>