[MOVEONLY] Move LastCommonAncestor to chain

src/chain.cpp

@@ -148,3 +148,22 @@ int64_t GetBlockProofEquivalentTime(const CBlockIndex& to, const CBlockIndex& fr
     }
     return sign * r.GetLow64();
 }
+
+/** Find the last common ancestor two blocks have.
+ *  Both pa and pb must be non-NULL. */
+const CBlockIndex* LastCommonAncestor(const CBlockIndex* pa, const CBlockIndex* pb) {
+    if (pa->nHeight > pb->nHeight) {
+        pa = pa->GetAncestor(pb->nHeight);
+    } else if (pb->nHeight > pa->nHeight) {
+        pb = pb->GetAncestor(pa->nHeight);
+    }
+
+    while (pa != pb && pa && pb) {
+        pa = pa->pprev;
+        pb = pb->pprev;
+    }
+
+    // Eventually all chain branches meet at the genesis block.
+    assert(pa == pb);
+    return pa;
+}