grunt jshint file filtering fails with long filenames

[atimmer]

"grunt jshint:core --file=password-strength-meter.js" shows more than only password-strength-meter.js because other paths for other files are the same length as the whole filename.

Attached patch fixes this by making sure indexOf is not -1.

[azaozz]

In 26168:

Grunt jshint: ensure the file passed with --file=filename.js matches all or part of the filepath before checking string lengths. Makes it possible to pass a file with full or partial path. All of these would work properly:
admin-bar.js, wp-includes/js/admin-bar.js, src/wp-includes/js/admin-bar.js. Props atimmer, fixes #25992.