QHash: improve perf when detaching with potential growth

By not assuming we will have resized.
reserve(size() + 1) will have suffered a bit from this, but
that is probably not very common.

Change-Id: I8750d430f532a72729ed06e67c0f315707a916d6
Reviewed-by: Lars Knoll <lars@knoll.priv.no>

src/corelib/tools/qhash.h

@@ -584,7 +584,7 @@ struct Data
         numBuckets = GrowthPolicy::bucketsForCapacity(qMax(size, reserved));
         spans = allocateSpans(numBuckets).spans;
         size_t otherNSpans = other.numBuckets >> SpanConstants::SpanShift;
-        reallocationHelper(other, otherNSpans, true);
+        reallocationHelper(other, otherNSpans, numBuckets != other.numBuckets);
     }
 
     static Data *detached(Data *d)