Add warning message when .git directory is not selected

**Steps to reproduce:**

1: When selecting a directory, select the repo folder itself and not the (hidden) `.git` folder within.

2: This leads to a seemingly "endless" loop where 'Loading ..' never ends.

**Motivation:**
As a new user of the app, I was skimming the documentation and didn't read the fine print about selecting the `.git` folder inside the repository. (I do think the documentation is good and well written; I just happened to miss it).

If you can realistically only select the .git folder (or subfolders), then some notification/warning would make you understand why the repo is not being loaded properly when selecting the wrong folder.