Fix AutoloadGenerator to work on MediaWiki-Vagrant

The use of realpath() in AutoloadGenerator::readFile() causes the
LocalSettings.php symlink to be dereferenced. Since the target file
lives outside of $IP, AutoloadGenerator would fail with an exception.

Change-Id: I4623b3da9b984026999189d70349ffb4754812a5
Merge "Fix AutoloadGenerator to work on MediaWiki-Vagrant"