cargo flapping the heck dependency

cargo seems to like to change heck back and forth between 0.4 and 0.5 for no reason.

You only notice this if the lockfile needs to be updated for some other reason.

I don't think this is a bug in d-d-git. I searched upstream cargo tickets and found only closed tickets

• https://github.com/rust-lang/cargo/issues/3043 (where one of the maintainers handwaves vaguely about path dependencies)
• https://github.com/rust-lang/cargo/issues/4312 (where one of the maintainers suggests that this is expected(!) but also says pls file ticket if it happens again)

I should make a proper repro steps and file a ticket against cargo I think.