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.