Orphan branch issue

serie (@ `53956faf13471204060182418a84df1c027cdb97`) crashes on orphan branch: ``` ~/dev/someproject main > git checkout --orphan new_branch ~/dev/someproject new_branch > serie 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>...]' thread 'main' (88191882) panicked at src/git.rs:489:21: HEAD not found in `git show-ref --head` output note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace ```