Stand-in line marking where a diff body was clipped for display.
Written by compute_unified_diff and rendered as a truncated row. It is also
the signal that any counts recomputed from the body would be short — see
DiffMessage._recount, which returns None rather than a known-low number.
Match it with is_truncation_marker rather than by hand: the renderer and the
recount must agree, or a body that renders "diff truncated" is also counted as
if it were complete.
