Without favouring any particular naming choice, remember that a git repo can be cloned into any root directory of your choice:
git clone https://github.com/user/repo.git myDir
Here repo.git would be cloned into the myDir directory. 
So even if your naming convention for a public repo ended up to be slightly incorrect, it would still be possible to fix it on the client side.
That is why, in a distributed environment where any client can do whatever he/she wants, there isn't really a naming convention for Git repo.
(except to reserve "xxx.git" for bare form of the repo 'xxx')
There might be naming convention for REST service (similar to "Are there any naming convention guidelines for REST APIs?"), but that is a separate issue.