This implements RFC-3127[^1] on Cargo side.

[^1]: https://rust-lang.github.io/rfcs/3127-trim-paths.html

The rustc commit hash is required as we'll need it to remap sysroot path
to `/rustc/<commit-hash>`, as rustc bootstrap does.
See <https://github.com/rust-lang/rust/blob/c2ef3516/src/bootstrap/src/lib.rs#L1113-L1116>.

This is optional as the compiler it may be built without a Git repository.

trim-paths is shown as disabled as default in `Debug` impl.
Although this doesn't reflect the correct default for `-Ztrim-pthas`,
this is no critical as it is only for debugging,
and it's a bit tricky to make it more correct.