Type Alias GitBranchResolveRefFn 

pub type GitBranchResolveRefFn = fn(gitdir: &Path, ref_name: &str) -> Result<Option<String>, BackendError>;

Engine::push_all dispatch, second half: resolve one local ref to its SHA. Ok(None) when the ref does not exist (a fresh mem-repo legitimately has no __MEMSTEAD yet).