The ordering of path-based excerpts in multibuffers regressed with
#38744, because we changed the `path` field of `PathKey` to be a string
(from `std::path::Path`) and used the derived `Ord` implementation,
which doesn't agree with the path-based order of worktree traversals.
This PR fixes that by using `RelPath` for `PathKey`. Instead of using
`File::full_path`, which can be absolute, we always use `File::path` and
distinguish different worktrees using their ID.

Release Notes:

- N/A

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Co-authored-by: Lukas Wirth <me@lukaswirth.dev>