Use SHA256D64 in Merkle root computation

Zcash: Excluding change to `BlockWitnessMerkleRoot` as we don't have
SegWit (deploying non-malleable txs via ZIP 244 instead).

(cherry picked from commit bitcoin/bitcoin@1f0e7ca09c)

src/bench/merkle_root.cpp

@@ -18,7 +18,7 @@ static void MerkleRoot(benchmark::State& state)
     }
     while (state.KeepRunning()) {
         bool mutation = false;
-        uint256 hash = ComputeMerkleRoot(leaves, &mutation);
+        uint256 hash = ComputeMerkleRoot(std::vector<uint256>(leaves), &mutation);
         leaves[mutation] = hash;
     }
 }

src/consensus/merkle.cpp

@@ -126,10 +126,23 @@ static void MerkleComputation(const std::vector<uint256>& leaves, uint256* proot
     if (proot) *proot = h;
 }
 
-uint256 ComputeMerkleRoot(const std::vector<uint256>& leaves, bool* mutated) {
-    uint256 hash;
-    MerkleComputation(leaves, &hash, mutated, -1, NULL);
-    return hash;
+uint256 ComputeMerkleRoot(std::vector<uint256> hashes, bool* mutated) {
+    bool mutation = false;
+    while (hashes.size() > 1) {
+        if (mutated) {
+            for (size_t pos = 0; pos + 1 < hashes.size(); pos += 2) {
+                if (hashes[pos] == hashes[pos + 1]) mutation = true;
+            }
+        }
+        if (hashes.size() & 1) {
+            hashes.push_back(hashes.back());
+        }
+        SHA256D64(hashes[0].begin(), hashes[0].begin(), hashes.size() / 2);
+        hashes.resize(hashes.size() / 2);
+    }
+    if (mutated) *mutated = mutation;
+    if (hashes.size() == 0) return uint256();
+    return hashes[0];
 }
 
 std::vector<uint256> ComputeMerkleBranch(const std::vector<uint256>& leaves, uint32_t position) {
@@ -158,7 +171,7 @@ uint256 BlockMerkleRoot(const CBlock& block, bool* mutated)
     for (size_t s = 0; s < block.vtx.size(); s++) {
         leaves[s] = block.vtx[s].GetHash();
     }
-    return ComputeMerkleRoot(leaves, mutated);
+    return ComputeMerkleRoot(std::move(leaves), mutated);
 }
 
 std::vector<uint256> BlockMerkleBranch(const CBlock& block, uint32_t position)

src/consensus/merkle.h

@@ -12,7 +12,7 @@
 #include "primitives/block.h"
 #include "uint256.h"
 
-uint256 ComputeMerkleRoot(const std::vector<uint256>& leaves, bool* mutated = NULL);
+uint256 ComputeMerkleRoot(std::vector<uint256> hashes, bool* mutated);
 std::vector<uint256> ComputeMerkleBranch(const std::vector<uint256>& leaves, uint32_t position);
 uint256 ComputeMerkleRootFromBranch(const uint256& leaf, const std::vector<uint256>& branch, uint32_t position);