fix: print warns when its an empty repo

[fundon]

#179

[edouard-lopez]

Thanks!

I was incorrect about the reason we didn't had the bug it test. The bug happen when cloning a bare repo, i.e. without any branch checkout, I was doing a git init in the test hence checking out master. See test file:

https://github.com/rafaelrinaldi/pure/blob/3aa9ea83d326e151f6325781228af6c3bfcd98f7/tests/_pure_prompt_git_pending_commits.test.fish#L61-L72