Auto merge of #142667 - yotamofek:pr/rustdoc/more-write-shared-perf, r=nnethercote

Avoid a few more allocations in `write_shared.rs`

Inspired by rust-lang/rust#141421 , avoids a few `Vec`, `PathBuf` and `String` allocations in `write_shared.rs`. I don't think these will show up on benchmarks, but are still worthwhile IMHO.
Also includes a few small cleanups.
r? nnethercote - if you'd like :)

Author: bors