fix: scm make patterns relative (#3996)
Since the Go `ChangedFiles` used the command line `git` it appears that
the CLI did some light data processing and if we passed the absolute
repo root as the pathspec via CLI we wouldn't have issue. It looks like
`libgit2` doesn't do this and expects well formed pathspecs so we
shouldn't be passing in absolute paths.
This PR will attempt to remove the repo root prefix from `relative_to`.
An alternative would be to change Go's behavior to no longer pass the
repo root as the default, but that's a larger change and the Rust and Go
implementations of `ChangedFiles` would still differ in the same way,
we'd just avoid hitting this behavior in turbo.
Test plan:
`GO_TAG=rust make integration-tests` now passes as well as the new unit
test
