mq: fix corner cases for handling of patch 0 in qselect Most of the code paths in mq would always pass patch specifications as a string. Patches can be specified by their index, but one code path passed that (through pop) to lookup as an integer - all other code paths used a string. Unfortunately pop and lookup (like many other parts of mq) used the boolean value of the patch specification to see if it was None, and they would thus incorrectly handle patch 0 as None. This patch makes the code comply with the actual internal duck typing of patch specifications: patch indices must be encoded as strings. The (now) unused code for partial and thus incorrect handling of indices as integers is removed.