Paste this into your console to enable click-to-copy on GitHub.com file paths

github-copy-file-path.js

(function(){
    $('.file-info[title], .file-info [title]')
    .css('cursor', 'pointer')
    .click(function() {
        var range = document.createRange();
        $(this).html($(this).attr('title'));
        range.selectNode(this);
        window.getSelection().addRange(range);
        try {
            var successful = document.execCommand('copy');
            var msg = successful ? 'successful' : 'unsuccessful';
            console.log('Copying', range.toString(), 'was', msg);
        } catch(err) {
            console.log('Oops, unable to copy');
        }
        window.getSelection().removeAllRanges();
    });
})();

You can sue http://mrcoles.com/bookmarklet/ to generate a bookmarklet as well