checkout: retire --ignore-other-worktrees in favor of --force

As a safeguard, checking out a branch already checked out by a different
worktree is disallowed. This behavior can be overridden with
--ignore-other-worktrees, however, this option is neither obvious nor
particularly discoverable. As a common safeguard override, --force is
more likely to come to mind. Therefore, overload it to also suppress the
check for a branch already checked out elsewhere.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>