Introduce test for checkout case-changing rename
Always checkout with case sensitive iterator

On a case-insensitive filesystem, we need to deal with case-changing
renames (eg, foo -> FOO) by removing the old and adding the new,
exactly as if we were on a case-sensitive filesystem.

Update the `checkout::tree::can_cancel_checkout_from_notify` test, now
that notifications are always sent case sensitively.
Merge pull request #2772 from ethomson/case_changing_rename

Case changing rename