Fix diff lines that were never rendered

[SimonCropp]

Fixes #7

Four defects caused changes to be silently omitted from the review output:

• Empty lines rendered as nothing. The word wrap calculation produced a piece count of zero for empty text, so the render loop never ran and an added or removed blank line was invisible.
• The last line of a snapshot could never render, because the range Stop is exclusive but was clamped to New.Count - 1.
• Changes running to the end of a snapshot were dropped entirely. Ranges were only flushed upon encountering a following unchanged line, so a change with no trailing unchanged line was never emitted.
• Line numbers drifted after the first insertion or deletion, since both columns used the index into the aligned diff rather than the position of each side within its own snapshot.

[patriksvensson]

LGTM 👍