Make `gix::diff::tree::visit::Action` more like `gix_traverse::tree::visit::Action`

[JacquesHumule]

Summary 💡

Permit to skip path in traversal for gix::diff::tree::Visit

Motivation 🔦

Enabling skipping in the diff visitor could reduce computation time, especially for scoped diff scenarios (e.g., git log <path>)

[Byron]

Indeed, it's a major shortcoming, which thus far could be worked around pretty well.
What's really needed here is full pathspec support. The engine for that exists, but the matching would need special care, particularly so that performance is as good as it can be in all cases.