git-remote-hg: fix compatibility with mercurial 3.2

[Mercurial 3.2 breaks git-remote-hg](https://github.com/felipec/git-remote-hg/issues/27). Upstream doesn't seem particularly active right now and fingolfin's fix seems pretty trivial (and works for me(tm)), so I think we should just temporarily pull it in until upstream gets around to it.

This PR switches over to fingolfin's fixed branch for now, but I can also turn it into a patch we apply to the upstream repo instead, if that's preferable.