make Configuration#Get methods virtual

The commit
https://github.com/libgit2/libgit2sharp/commit/3d29d95dc7d0addc889679c93d22952fe7bb9d35
moved #Get overloads to extension methods. The problem is the remaining
#Get methods aren't virtual, which means you can't stub in unit tests.
Which broke a bunch of GHfW tests.