range: Properly compare ranges

The behavior of gcal_range_compare(), and the 'position' argument
of gcal_range_calculate_overlap(), should be the same. However,
because we were comparing the end dates in the wrong order, they
didn't match.

Fix that by properly comparing the end of the ranges.

src/core/gcal-range.c

@@ -422,7 +422,7 @@ gcal_range_compare (GcalRange *a,
   result = compare_func (a->range_start, b->range_start);
 
   if (result == 0)
-    result = compare_func (b->range_end, a->range_end);
+    result = compare_func (a->range_end, b->range_end);
 
   return result;
 }