Prune paths when using --full-path and a pattern containing the start of string anchor

[dustinpho]

I recently ran into a scenario where fd could be improved. If the pattern contains the start-of-string (^), fd will continue trying all entries under a subdirectory even if its parent does not 'partially match' the pattern. I ran into a situation where it took several minutes to try to find some files using fd whereas find finished in a few seconds (I assume because it does support pruning with a regex that contains start-of-string -- I did not verify this by looking at the source yet).

The fd command that I had used to compare the performance between the two was:
fdfind <regex> <root-dir> --full-path
and the find command was:
find <root-dir> -regex <regex>
where regex could look like:
^/mnt/foo/bar/([^/]*)/([^/]*)/(baz|qux)/...

Note: This is different than --prune as that option only skips the subdirectory tree if there's a match.

The ideal behavior here is to skip the subdirectory tree if it's impossible to have a match (because there's no partial match) only when matching against full path. If there is not a partial regex match for any of the patterns when we're trying to matching against an full path, then prune (return ignore::WalkState::Skip).

FYI I don't believe the Rust regex library current supports partial matching (rust-lang/regex#678).