function getRelativeFilename(basePath, filename) {
        if (!filename || filename.substr(0, basePath.length) !== basePath) {
            return;
        }

        return filename.substr(basePath.length);
    }
Computes filename as relative to the basePath. For example:
basePath: /foo/bar/, filename: /foo/bar/baz.txt
returns: baz.txt

The net effect is that the common prefix is stripped away. If basePath is not
a prefix of filename, then undefined is returned.

@param {string} basePath Path against which we're computing the relative path
@param {string} filename Full path to the file for which we are computing a relative path
@return {string} relative path