Change depth - 1 to newDepth

Replacing 'depth - 1' with 'newDepth' in the singularbeta formula
utilizes existing variables more succinctly.

closes official-stockfish#4876

No functional change

src/search.cpp

@@ -1045,7 +1045,7 @@ Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, boo
                 && tte->depth() >= depth - 3)
             {
                 Value singularBeta  = ttValue - (64 + 57 * (ss->ttPv && !PvNode)) * depth / 64;
-                Depth singularDepth = (depth - 1) / 2;
+                Depth singularDepth = newDepth / 2;
 
                 ss->excludedMove = move;
                 value =