Function fetch_and_rebase_target 

pub fn fetch_and_rebase_target(
    base_branch: &str,
    repo: &Path,
    cwd: &Path,
) -> (bool, String)

Fetch from remote and determine the rebase target for a base branch.

Returns (fetch_ok, rebase_target) where rebase_target is origin/<base> if fetch succeeded and the remote ref exists, otherwise just <base>.