Increase reduction in case of stable best move

The idea of this patch is pretty simple - we already do more reductions
for non-PV and root nodes in case of stable best move for depth > 10.
This patch makes us do so if root depth if > 10 instead, which
is logical since best move changes (thus instability of it) is
counted at root, so it makes a lot of sense to use depth of the root.

passed STC
https://tests.stockfishchess.org/tests/view/5fd643271ac16912018885c5
LLR: 2.94 (-2.94,2.94) {-0.25,1.25}
Total: 13232 W: 1308 L: 1169 D: 10755
Ptnml(0-2): 39, 935, 4535, 1062, 45

passed LTC
https://tests.stockfishchess.org/tests/view/5fd68db11ac16912018885f0
LLR: 2.96 (-2.94,2.94) {0.25,1.25}
Total: 14024 W: 565 L: 463 D: 12996
Ptnml(0-2): 3, 423, 6062, 517, 7

closes #3263

Bench: 4050630

src/search.cpp

@@ -1188,7 +1188,7 @@ namespace {
  r -= 2;
 
  // Increase reduction at root and non-PV nodes when the best move does not change frequently
- if ((rootNode || !PvNode) && depth > 10 && thisThread->bestMoveChanges <= 2)
+ if ((rootNode || !PvNode) && thisThread->rootDepth > 10 && thisThread->bestMoveChanges <= 2)
  r++;
 
  // More reductions for late moves if position was not in previous PV