elvish uses lexical scoping for closure capture, as such all `oldpwd`
references within the script refers to the `oldpwd` defined at the top.
However, before-chdir hook is trying to assign to `oldpwd` within the
editor scope, which is not used by this script.
This manifested as a bug in which:
```
~
> mkdir -p /tmp/another
~
> z /tmp
/tmp
> z another
/tmp/another
> z -
~
> # The previous dir should be /tmp not ~!
```
Because the hook was updating a variable that was not used.
Fix the hook so that before-chdir assign to the proper upvalue.
