completion: add remaining flags to checkout

completion: add remaining flags to checkout

In the commits 1fc458d9 (builtin/checkout: add --recurse-submodules
switch, 2017-03-14), 08d595dc (checkout: add --ignore-skip-worktree-bits
in sparse checkout mode, 2013-04-13) and 32669671 (checkout: introduce
--detach synonym for "git checkout foo^{commit}", 2011-02-08) checkout
gained new flags but the completion was not updated, although these flags
are useful completions. Add them.

The flags --force and --ignore-other-worktrees are not added as they are
potentially dangerous.

The flags --progress and --no-progress are only useful for scripting and are
therefore also not included.

Signed-off-by: Thomas Braun <thomas.braun@virtuell-zuhause.de>
Reviewed-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>