`nimble run` fails inside git repo with no commits
## How to reproduce
1. Create a directory
2. Run `git init`
3. Run `nimble init -y` (with fix from https://github.com/nim-lang/nimble/pull/983)
4. Run `nimble run`

```
    Error:  Execution of 'git -C /tmp/db rev-parse HEAD' failed with an exit code 128.
        ... Details: fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
        ... Use '--' to separate paths from revisions, like this:
        ... 'git <command> [<revision>...] -- [<file>...]'
        ... HEAD
```

Why does nimble need to use git anyway?