sqlps's provider causes posh-git to throw errors

When doing something like importing the SQL Server 2012 powershell extensions, you will get errors from Posh-git if you're in that SQLSERVER provider. This doesn't affect most providers because most of them return quietly when something goes wrong. But SQLSERVER provider in particular does a WMI query any time you `cd` in to something.

The error is similar to:
![image](https://cloud.githubusercontent.com/assets/936215/4328117/2ae86536-3f81-11e4-8962-c2850fa20ba9.png)

There's a couple of places this can be fixed, but I felt like the best place was in Get-LocalOrParentPath and having it check that the $checkIn object is on a provider named 'FileSystem'. I'm happy to fix this, but I wasn't sure if that's really the best place to fix the issue. Also testing a string vs. string may be brittle to locales and I got lucky - I'm honestly unsure; however, a cursory check of e.g. German showed it was still called 'FileSystem'.

Locally I have already fixed it by modifying it as mentioned and all is happy.