Globber Relative Path Fix

Fixes issue in Cake.Core.IO.Globber.Walk() resulting some relative
paths would result in below exception
startIndex cannot be larger than length of string.

This because the relative rootpath could be longer/different vs.
child file/folder paths.

I.e. this would fail:
..\cake\src\Cake\bin\Debug\Cake.exe .\build.csx -solution="..\Cake\src\Cake.sln"

Fix uses the PathCollapser.Collapse() to clean up the full root path
causing the issue.