start_head=''
        if test -s "$GIT_DIR/BISECT_START"
        then
                # Reset to the rev from where we started.
                start_head=$(cat "$GIT_DIR/BISECT_START")
                git checkout "$start_head" || exit
        else
                # Get rev from where we start.
                case "$head" in
                refs/heads/*|$_x40)
                        # This error message should only be triggered by
                        # cogito usage, and cogito users should understand
                        # it relates to cg-seek.
                        [ -s "$GIT_DIR/head-name" ] &&
                                die "won't bisect on seeked tree"
                        start_head="${head#refs/heads/}"
                        ;;
                *)
                        die "Bad HEAD - strange symbolic ref"
                        ;;
                esac