depth: Limit black 3rd move thinking depth to less than 3

For this move list:

 1.     b4    f4
 2.     d6    d2
 3.     b6    b2
 4.     f6xb2    b2
 5.     c5

Black:
Bad move:  f2xb4 (lost in 39)
Good move: f2xc5/ or no mill

(From so_gewinnt_man_muehle)

We cannot stop black to do bad move at 5th.
So we try to stop black to do b2 at 4th.
Through change black's 3rd move depth to 3,
Black will do d7/d5/a4/c4.

This commit is not conflict with
"depth: Limit 6th and 7th move depth to 15"
(fefa064cb9)

Test result:
If change depth to less than 2 (not 3), played 600-800 games.
Before change:
20% : 32% : 48%
After change:
17% : 43% : 39%

src/mills.cpp

@@ -470,7 +470,7 @@ Depth get_search_depth(const Position *pos)
         if (pos->phase == Phase::placing) {
             const Depth placingDepthTable[25] = {
                   +1,  1,  +1,  1,   /* 0 ~ 3 */
-                  +3, 15, +15, 15,   /* 4 ~ 7 */
+                  +3, 3, +15, 15,   /* 4 ~ 7 */
                   +15, 2, +8,  0,    /* 8 ~ 11 */
                   +0, 0, +0,  0,     /* 12 ~ 15 */
                   +0, 0, +0,  0,     /* 16 ~ 19 */