Bug 22461: Clarify and correct the hash reduction

Bug 22461: Clarify and correct the hash reduction

There were a couple of bugs in the previous patch which meant it wasn't
working as intended. This patch corrects those bugs and simplifies the
code a little along the way.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Alex Arnaud <alex.arnaud@biblibre.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
(cherry picked from commit c9fa30a55b585b3351ece9c3f789fd18d9807fac)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>