rpc: Compute number of confirmations of a block from block height

Currently this uses a CMerkleTx, but that makes no sense as we
have the CBlockIndex available. As noted by @jgarzik.

src/rpcblockchain.cpp

@@ -54,9 +54,11 @@ Object blockToJSON(const CBlock& block, const CBlockIndex* blockindex)
 {
     Object result;
     result.push_back(Pair("hash", block.GetHash().GetHex()));
-    CMerkleTx txGen(block.vtx[0]);
-    txGen.SetMerkleBranch(&block);
-    result.push_back(Pair("confirmations", (int)txGen.GetDepthInMainChain()));
+    int confirmations = -1;
+    // Only report confirmations if the block is on the main chain
+    if (chainActive.Contains(blockindex))
+        confirmations = chainActive.Height() - blockindex->nHeight + 1;
+    result.push_back(Pair("confirmations", confirmations));
     result.push_back(Pair("size", (int)::GetSerializeSize(block, SER_NETWORK, PROTOCOL_VERSION)));
     result.push_back(Pair("height", blockindex->nHeight));
     result.push_back(Pair("version", block.nVersion));
@@ -242,7 +244,7 @@ Value getblock(const Array& params, bool fHelp)
             "\nResult (for verbose = true):\n"
             "{\n"
             "  \"hash\" : \"hash\",     (string) the block hash (same as provided)\n"
-            "  \"confirmations\" : n,   (numeric) The number of confirmations\n"
+            "  \"confirmations\" : n,   (numeric) The number of confirmations, or -1 if the block is not on the main chain\n"
             "  \"size\" : n,            (numeric) The block size\n"
             "  \"height\" : n,          (numeric) The block height or index\n"
             "  \"version\" : n,         (numeric) The block version\n"