Change loop condition in minRunLength (fixes #33)

For some reason the loop condition in minRunLength was MIN_MERGE=32
while the original CPython implementation uses 64 there. This commit
changes that condition to 2*MIN_MERGE to bring the condition back to 64
at it should probably have been.

Thanks @weeyizhi for identifying and the fix.

Author: Morwenn

include/gfx/timsort.hpp

@@ -171,7 +171,7 @@ template <typename RandomAccessIterator, typename Compare> class TimSort {
         GFX_TIMSORT_ASSERT(n >= 0);
 
         diff_t r = 0;
-        while (n >= MIN_MERGE) {
+        while (n >= 2 * MIN_MERGE) {
             r |= (n & 1);
             n >>= 1;
         }