regexpfs: always use file name instead of path

[satotake]

On RegexpFs, some operations apply given regexps with not file name but file path. Fix this.

README

A filtered view on file names, any file NOT matching the passed regexp will be treated as non-existing.

[jxsl13]

according to the description of RegexpFs this fix makes sense.

(imo RegexpFs should be more powerful to also match full file paths but that's another discussion)