Fix a crash on Windows when trying to clone/create a repository from a root folder (D:\)

This is because Environment.GetCommandLineArgs() sometimes messes up the arguments, and in this particular case, transforms "D:\" into D:", which is invalid both because a quote is not a valid character inside a path, and because the trailing slash after D: is mandatory.

The problem is not inside the Shell Extension. I checked:
![shellext](https://cloud.githubusercontent.com/assets/1535823/5157914/45bf46be-7329-11e4-99ea-0c18389d47fa.png)


(I hope the base branch is the right one. I could not find an older one than 2.48 even though https://raw.githubusercontent.com/gitextensions/gitextensions/configdata/GitExtensions.releases tells me I should use 2.46 ?)