Enforce `.mailmap` correctness in tidy

[albertlarsan68]

@Mark-Simulacrum said, in rust-lang/thanks#2 (comment):

We would at least like to validate the mailmap in rust-lang/rust.

Then, at rust-lang/thanks#2 (comment):

Ah, I would still like to eventually add the validation to tidy, but that doesn't need to be tracked here.

I did not see any issue corresponding to that, so here it is.