Problem:
- buf-local CWD scope is lower priority than :lcd, which is weird.
```
win > buf > tab > global
```
- No way to clear current CWD at a given scope.
Solution:
- Rerrange scope precedence to:
```
buf > win > tab > global
```
- Introduce "bang" variants (`:bcd!`/`:lcd!`/`:tcd!`) which clears the
local CWD for the given scope.