Function detect_backend 

pub fn detect_backend(
    repo_path: &Path,
    vcs_override: Option<VcsOverride>,
) -> Result<Box<dyn VcsBackend>, Error>

Detect the appropriate VCS backend for a repository.

If vcs_override is Some(VcsOverride::Git), always uses git. Otherwise, checks for .jj directory first (colocated repos), then falls back to git.