turning an existing dir into a git repo

current code assumes that the target student dir

* either does not exist
* or it is a git repo

in my use case, for transisioning purposes primarily, I have to consider the case where the directory exists but is not yet a git repo; typically the students have started to work on notebooks obtained via email or other distribution schemes

adding code for taking care of this situation looks easy, and it seems easy to ensure that would not break anything