Question: `git mv` renames a file or directory in a repository. How do I rename the Git repository itself?

Answer: 
Rename `PRJ0.git` to `PROJ1.git`, then edit the URL variable located in the `.git/config` file of your project.
Git itself has no provision to specify the repository name. The root directory's name is the single source of truth pertaining to the repository name.

The `.git/description` though is used only by some applications, like [GitWeb](https://git-scm.com/book/en/v2/Git-on-the-Server-GitWeb).