Throw more helpful exception when directory not a git repository

[csmager]

When findGitDir reaches the root folder having not found a .git directory, we throw an InvalidOperationException with the message:

"Not a git repository: no .git directory found in the specified directory or any parent directory."

This should be a little more helpful than the previous NullReferenceException, but doesn't change the API.

Fixes #2249

[matthid]

Thanks a lot! Looks good to me.