Support local content source with no commits

Antora should be able to use a local content source repository even if it contains no commits. A repository is in this state immediately after the call to git init .. As long as one of the entries in the branches pattern of the content source is HEAD, Antora should be able to use the worktree from this repository. This is a convenient way to quickly enable a local directory to be used with Antora without having to interact with git.