"ignore" crate: Parallel walker should ignore broken symlinks

[ruffe972]

This fixes sharkdp/fd#746. "fd": uses "ignore" crate from this repo. Steps to reproduce:

• Create parallel walker with "follow links" option enabled.
• Create broken symlink with the name broken_symlink, for example.
• Add "broken_symlink" to ignored files.
• Get walker dir entry results.

Expected: walker doesn't return dir entry result for the link.
Actual: walker returns error result for the link.

Note that I didn't fix similar bug in single-threaded walker. I'm new to Rust so it's a bit difficult for me, but if it's required for this PR to be merged, I'll fix single-threaded walker too.

[BurntSushi]

I've opened #2552 for this. I'm not sure this is the right way to fix this issue, and the fact that the single threaded case isn't fixed means I don't want to bring this in as-is. I looked and fixing the single threaded case appears tricky.

I'm going to close this for now in favor of tracking #2552 as a bug. Since I don't consider this to be a high priority bug, I'm going to defer fixing this until I've had a chance to overhaul ignore's internals.