Detect symlinks with infinite loops

[Kulgar]

As asked in the issue #217 I'm opening a new issue.

Here is the issue I found:

A symlink file was linking to the root folder of my application. Thus, guard/listen was following this file again and again, creating an infinite loop.

You can easily create such a symlink, for instance in your vendor folder, add a symlink to ".." (the parent directory). When you launch guard/listen it should follow this symlink again and again.

I discovered this issue while developing on Canvas LMS, you can find the concerned symlink file here: https://github.com/instructure/canvas-lms/blob/master/client_apps/canvas_quiz_statistics/vendor/canvas

Thanks,
Kulgar.

[e2]

Fixed in 2.8 by #273