walker-skip should accept nested directories, not just a single path component

[jyn514]

Checklist

• [X] I have read through the manual page (man fzf)
• [X] I have searched through the existing issues
• [X] For bug reports, I have checked if the bug is reproducible in the latest version of fzf

Output of fzf --version

0.56.3 (add1aec)

OS

• [X] Linux
• [ ] macOS
• [ ] Windows
• [ ] Etc.

Shell

• [ ] bash
• [X] zsh
• [ ] fish

Problem / Steps to reproduce

$ mkdir -p a/b
$ touch a/b/foo
$ fzf --walker-skip a/b

i expect that this does not return a/b/foo. instead it does show it.

if i use --walker-skip b, it correctly ignores the file. but i don't want to ignore all directories named b, only this particular directory.