Add warning against Unicode Bidi characters in our range-diff

[Urgau]

Like GitHub, let's show a warning if we detect bidirectional characters in the patch.

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Links:

• https://github.co/hiddenchars
• rust-lang/rust@c0b1345
• http://localhost:8000/gh-range-diff/rust-lang/rust/38b01d9..38b01d9/38b01d9..c0b134582a949062f9f3a8ba3def88b98a98df6a

Suggested by @camelid

[Kobzol]

Not sure if I'm a fan of adding a new dependency (and more code) for something so niche, tbh. But I won't block it.

View changes since this review

[Urgau]

It could be done without the dependency on memchr, but since we already have that dependency 18 times in our Cargo.lock, I think it's fine.

As for adding something this "niche", I would agree at first, but since GitHub decided to add such warning, that we had a rustc CVE for it (CVE-2021-42574), and that we added a deny-by-default lint for it (text_direction_codepoint_in_literal). I don't think it's that niche.

[camelid]

Thanks @Urgau!