Fix xv to hit % when wielding sniper, and always bypass shield

Recent to hit changes introduced a bug when hit
chance is AUTOMATIC_HIT causing the % to hit to
display as 1%.

While fixing this I noticed there is a very tiny (but still
non-zero) chance for AUTOMATIC_HIT attacks to
be blocked by a shield. Since this is either unexpected
or should be rather more significant, it seems
easiest to just bypass shield checks as well for
something like Sniper.

crawl-ref/source/attack.cc

@@ -1098,7 +1098,7 @@ int attack::apply_defender_ac(int damage, int damage_max, ac_type ac_rule) const
  */
 bool attack::attack_shield_blocked(bool verbose)
 {
-    if (defender == attacker)
+    if (defender == attacker || to_hit >= AUTOMATIC_HIT)
         return false; // You can't block your own attacks!
 
     // Divine Shield blocks are guaranteed, no matter what.

crawl-ref/source/fight.cc

@@ -89,18 +89,12 @@ int aux_to_hit()
 
 }
 
-static double _to_hit_to_land(attack &atk)
-{
-    int to_land = atk.calc_pre_roll_to_hit(false);
-    if (to_land >= AUTOMATIC_HIT)
-        return 1;
-
-    return to_land;
-}
-
 static double _to_hit_hit_chance(const monster_info& mi, attack &atk, bool melee,
                                  int to_land, bool is_aux = false)
 {
+    if (to_land >= AUTOMATIC_HIT)
+        return 1.0;
+
     const double AUTO_MISS_CHANCE = is_aux ? 0 : 2.5;
     const double AUTO_HIT_CHANCE = is_aux ? 3.3333 : 2.5;
 
@@ -156,6 +150,10 @@ static double _to_hit_shield_chance(const monster_info& mi,
     // Duplicates more logic that is defined in attack::attack_shield_blocked, and
     // attack_melee and attack_ranged classes, for real attacks.
 
+    // Guaranteed hits should ignore the shield as well
+    if (to_land >= AUTOMATIC_HIT)
+        return 0;
+
     // Attack first checks for incapacitation, this is handled with a shield bonus
     // of -100 (or if they have no shield) so we can resolve this here.
     if (mi.shield_bonus == -100)
@@ -187,7 +185,7 @@ static double _to_hit_shield_chance(const monster_info& mi,
 int to_hit_pct(const monster_info& mi, attack &atk, bool melee,
                bool penetrating, int distance)
 {
-    const int to_land = _to_hit_to_land(atk);
+    const int to_land = atk.calc_pre_roll_to_hit(false);
     const double hit_chance = _to_hit_hit_chance(mi, atk, melee, to_land);
     const double shield_chance = _to_hit_shield_chance(mi, melee, to_land, penetrating);
     const int blind_miss_chance = player_blind_miss_chance(distance);