Add a RepoPath struct.

So we don't have to remember whether a path is absolute or relative to
the repo root.

The downside is a lot of copies of the repo root floating around, but
this is not performance-critical code.

Test: cargo run --bin health_report ; cargo run --bin migration_report
Bug: 339424309
Change-Id: I86805b5fdb0d43bc71162dee605c65f0643da130
9 files changed