use buffer-file-name to filter buffer list

This makes the function compatible with certain uniquify-buffer-name configurations.. 
An example file name might be "layout.scss • Resources/static/shared".
I don't know if there might be a reason to not match against the buffer-file-name, I can change the code to check for both if you want to.

One thing I dont understand about the code is why a buffer name should not start with s- and if that should be checked against the buffer-name instead of buffer-file-name.