CPK fails when there is no git HEAD

```
cpk| INFO : CPK - Code Packaging toolKit - v0.1.1
fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git [...] -- [...]'
Traceback (most recent call last):
File "/home/ripl/anaconda3/bin/cpk", line 6, in
run()
File "/home/ripl/anaconda3/lib/python3.8/site-packages/cpk/cli/main.py", line 66, in run
command.execute(machine, parsed)
File "/home/ripl/anaconda3/lib/python3.8/site-packages/cpk/cli/commands/build.py", line 126, in execute
project = CPKProject(parsed.workdir, parsed=parsed)
File "/home/ripl/anaconda3/lib/python3.8/site-packages/cpk/project.py", line 46, in __init__
30: GitRepositoryAdapter(self._path),
File "/home/ripl/anaconda3/lib/python3.8/site-packages/cpk/adapters/git.py", line 13, in __init__
self._repo = get_repo_info(self._path)
File "/home/ripl/anaconda3/lib/python3.8/site-packages/cpk/utils/git.py", line 16, in get_repo_info
sha = run_cmd(["git", "--git-dir", f'"{path}"',
File "/home/ripl/anaconda3/lib/python3.8/site-packages/cpk/utils/misc.py", line 17, in run_cmd
lines = subprocess.check_output(cmd, shell=True).decode("utf-8").split("\n")
File "/home/ripl/anaconda3/lib/python3.8/subprocess.py", line 411, in check_output
return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
File "/home/ripl/anaconda3/lib/python3.8/subprocess.py", line 512, in run
raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command 'git --git-dir "/home/ripl/code/test-project/.git" rev-parse HEAD' returned non-zero exit status 128.
```