html-dir: In html-dir output, split large diffs across several child pages

Also add some flags so html-dir behaviour is easier to reason about.

1. Previously, max-report-size would apply to both parent and child pages,
which didn't work well if you disabled this limit.

Instead, we now add an extra --max-report-child-size that remains in effect
even with --no-default-limits.

2. Previously, separate-file-diff-size would affect the parent page in an
unsmooth way: if a diff block had N lines, the parent page would contain N
lines, but if the diff block had N+1 lines, then 0 would be shown on the parent
page. This gives a visually counterintuitive result where a larger diff would
have less presence on the parent overview page.

Instead, we now change this to --max-diff-block-lines-parent which has a
smoother behaviour. If the diff block has N+1 lines, then N would be shown on
the parent page and 1 would be shown on the child page. This could be smoothed
out further, but we'll leave this complexity for the future.