Support for `HEAD`, `WORKING` and `STAGED` in fully-qualified views

Today, if I have a view like this...
```sql
create view v_diff_stat as
select * from dolt_diff_stat('main', 'WORKING');
```

...and select from the view using a fully-qualified name...
```sql
select * from `mysql/my-branch`.`v_diff_stat`;
```

...it will always ignore the branch name and use the checked-out branch instead. I think a fully-qualified name should have precedence, feels more intuitive, but I might be wrong. It would at least be pretty cool if the above worked.

Reproduce the SQL examples with a view calling dolt_diff_stat('main', 'WORKING') and a fully-qualified view name such as `mysql/my-branch`.`v_diff_stat`. Start by tracing view resolution and handling of HEAD, WORKING, and STAGED references; done means the qualified branch takes precedence and the view returns results for that branch.

Written by the indexing model from the issue text.