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

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

This reverts commit 0d1a1517835a10818f2d40d8780a268dbb5e20ce.

When trying to switch to a different branch, that happens to be
checked out in another working tree, the user shouldn't have to
give up the other safety measures (like protecting the local changes
that overlap the difference between the branches) while defeating
the "no two checkouts of the same branch" safety.

Signed-off-by: Junio C Hamano <redacted>