Auto merge of rust-lang#85538 - r00ster91:iterrepeat, r=Mark-Simulacrum

Replace some `std::iter::repeat` with `str::repeat`

I noticed that there were some instances where `std::iter::repeat` would be used to repeat a string or a char to take a specific count of it and then collect it into a `String` when `str::repeat` is actually much faster and better for that.

See also: rust-lang/rust-clippy#7260.

Author: bors