(bug 48212) Fix I720946e7: $this->opts['year'] shouldn't be modified.

Otherwise, the offset is set automatically based on the default 'year'
parameter, then ReverseChronologicalPager gets misled and assumes that
the user have already specified a starting point so there must be a
previous page, causing a 'prev' link returning nothing shown.

Bug: 48212
Bug: 45580
Change-Id: I5341ff8c0d0422b9b3184dce74f0f143822cfe93