D5503: vfs: add support for repo names with `$` when using with env vars (issue5739)

[Yuya Nishihara]

>   > This is logically incorrect. The problem is that we're doing variable
>   >  expansion at too lower layer. `vfs(expand(user_specified_path))` makes
>   >  some sense, but `vfs(expand(getcwd()))` is clearly wrong. And the vfs class
>   >  can't know where the `base` comes from.
>   
>   If I add a condition for expanding env var if present in `hgrc`, can this work as a fix?

Maybe no. Where do you intend to add such code? vfs doesn't know where the path
comes from. Neither does localrepo.

>   If the only solution for this is shifting path expansion from `vfs` class, where do you
>   think it can be?

Somewhere specifying repository path read from hgrc or user input.