tell mysql to ignore the sort index for search queries

mysql really likes to pick an index for sorting if it can't fully satisfy the where
filter with an index, since search queries pretty much never are fully filtered by index
mysql often picks an index for sorting instead of the *much* more useful index for filtering.

To bypass this, we tell mysql explicitly not to use the mtime (the default order field) index,
so it will instead pick an index that is actually useful.

Signed-off-by: Robin Appelman <robin@icewind.nl>