Excluding the boundaries of a one or two unit long period pushes the
included start past the included end. diff()->days is unsigned and the
HOUR/MINUTE/SECOND branch applied abs() outright, so that backwards range
was measured as a positive distance and the unconditional +1 inflated it
further: a one day period reported 1, 2, 2 and 3 for EXCLUDE_NONE, START,
END and ALL, while iterating the same objects yielded nothing.
Return 0 up front when the included start is past the included end. This
matches what the MONTH and YEAR precisions already returned through
iterator_count(), and lets the now unreachable abs() go.
