Optimizing Special:DisambiguationPages query to avoid filesort

Changing the sorting behavior for Special:DisambiguationPages so
that it won't use a filesort. This reduces the query time by a few
orders of magnitude. Tested on en.wiki slave db: Query now runs in
about 0.03 seconds.

Bug: 50832
Change-Id: I1664446ea409710d2542a0a60b0ccb3c4b9b867a
Merge "Optimizing Special:DisambiguationPages query to avoid filesort"