Change counts for diff, taken before it was truncated for display.
The single provenance for what a DiffMessage shows. None is the only
way this says "unknown" — never DiffStats(0, 0), which means a verified
zero. Unset until a diff is computed, and left unset for both outcomes that
have nothing to count: untrusted_before, where any count would be fiction,
and unreadable_after, which returns before a diff is ever computed.
Deliberately not metrics.lines_added/lines_removed, which are session
accounting and do not always mean diff lines: a new-file write_file sets
lines_added from the whole file rather than from a diff.
