Trivial optimization: use GetAncestor to compute new target

src/pow.cpp

@@ -42,9 +42,9 @@ unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHead
     }
 
     // Go back by what we want to be 14 days worth of blocks
-    const CBlockIndex* pindexFirst = pindexLast;
-    for (int i = 0; pindexFirst && i < params.DifficultyAdjustmentInterval()-1; i++)
-        pindexFirst = pindexFirst->pprev;
+    int nHeightFirst = pindexLast->nHeight - (params.DifficultyAdjustmentInterval()-1);
+    assert(nHeightFirst >= 0);
+    const CBlockIndex* pindexFirst = pindexLast->GetAncestor(nHeightFirst);
     assert(pindexFirst);
 
     return CalculateNextWorkRequired(pindexLast, pindexFirst->GetBlockTime(), params);