This change does more things in the parallel loop, refactors the file-level
logic into two functions for added clarity.
This bit of Rust code takes 55ms to execute on a repo where the stat'ing part
of Valentin's fast path takes 40ms. While the code differs a bit and it's hard
to get an exact measurement of how much of a performance impact it has, I can
be fairly certain that this implementation is *at worse* twice as slow.