Evaluate pathspecs in-process

[zombiezen]

While there would be some upfront complexity, there would be tremendous simplification of internals if gg were able to evaluate pathspecs in-process rather than having to depend on Git every time. Main hurdles are translating real paths into top-level paths and then correctly supporting all the options that Git supports.

[zombiezen]

Some progress: I can now reliably translate real paths into top-level paths. See worktreeRelativePath.