get_filenames() behavior

@dafriend thanks for the feedback! Sorry if my examples were unclear. See the third code block for what I mean by "relative" - it includes the portion of the path between the $source_dir you passed in and the filename. So if I pass in /foo/bar and the only file in there is nested at /foo/bar/bat/baz/file.txt the current behavior would return ['file.txt'], with $include_source it would return ['/foo/bar/bat/baz/file.txt'], and my proposed "relative" return would be ['bat/baz/file.txt'].