`nimble publish` fails if `init.defaultbranch` is set to something other than `master`
`nimble publish` (version 0.13.1) fails consistently with two Git errors:

```
     Error: Execution failed with exit code 1
        ... Command: git push https://{token}@github.com/vtbassmatt/packages master
        ... Output: error: src refspec master does not match any
        ... error: failed to push some refs to 'https://github.com/vtbassmatt/packages'
```

This seems to be because I have `init.defaultbranch` set to `main` in my global gitconfig. I worked around it by temporarily changing my global gitconfig.

I dug into the code to understand what's going on, and am a little stumped. I see [here](https://github.com/nim-lang/nimble/blob/6a2cf71a1754f5f39995e8311b831a32e0676435/src/nimblepkg/publish.nim#L182) that right after we `git init`, we attempt to create the desired default branch. In testing this manually (using the underlying Git commands), the behavior is what we expect; nevertheless, the bug repros consistently for me.

One possible fix: doing a straight `git clone` would give the correct behavior. Given that `nimble` is already writing down the token in the user's home directory, it doesn't seem any _more_ unsafe to let Git write it down as well.