do not write unchanged unit attributes

This should make savefiles smaller. Note that this does not effect all
games since some wml codes will disable this optimisation.

src/game_board.cpp

@@ -341,7 +341,7 @@ void game_board::write_config(config & cfg) const
 			if(i.side() == side_num) {
 				config& u = side.add_child("unit");
 				i.get_location().write(u);
-				i.write(u);
+				i.write(u, false);
 			}
 		}
 		//recall list

src/units/attack_type.cpp

@@ -60,7 +60,8 @@ attack_type::attack_type(const config& cfg) :
 	accuracy_(cfg["accuracy"]),
 	movement_used_(cfg["movement_used"].to_int(100000)),
 	parry_(cfg["parry"]),
-	specials_(cfg.child_or_empty("specials"))
+	specials_(cfg.child_or_empty("specials")),
+	changed_(true)
 {
 	if (description_.empty())
 		description_ = translation::egettext(id_.c_str());
@@ -213,6 +214,7 @@ bool attack_type::apply_modification(const config& cfg)
 	if( !matches_filter(cfg) )
 		return false;
 
+	set_changed(true);
 	const std::string& set_name = cfg["set_name"];
 	const t_string& set_desc = cfg["set_description"];
 	const std::string& set_type = cfg["set_type"];

src/units/attack_type.hpp

@@ -53,18 +53,18 @@ class attack_type : public std::enable_shared_from_this<attack_type>
 	double defense_weight() const { return defense_weight_; }
 	const config &specials() const { return specials_; }
 
-	void set_name(const t_string& value) { description_  = value; }
-	void set_id(const std::string& value) { id_ = value; }
-	void set_type(const std::string& value) { type_ = value; }
-	void set_icon(const std::string& value) { icon_ = value; }
-	void set_range(const std::string& value) { range_ = value; }
-	void set_accuracy(int value) { accuracy_ = value; }
-	void set_parry(int value) { parry_ = value; }
-	void set_damage(int value) { damage_ = value; }
-	void set_num_attacks(int value) { num_attacks_ = value; }
-	void set_attack_weight(double value) { attack_weight_ = value; }
-	void set_defense_weight(double value) { defense_weight_ = value; }
-	void set_specials(config value) { specials_ = value; }
+	void set_name(const t_string& value) { description_  = value; set_changed(true); }
+	void set_id(const std::string& value) { id_ = value; set_changed(true); }
+	void set_type(const std::string& value) { type_ = value; set_changed(true); }
+	void set_icon(const std::string& value) { icon_ = value; set_changed(true); }
+	void set_range(const std::string& value) { range_ = value; set_changed(true); }
+	void set_accuracy(int value) { accuracy_ = value; set_changed(true); }
+	void set_parry(int value) { parry_ = value; set_changed(true); }
+	void set_damage(int value) { damage_ = value; set_changed(true); }
+	void set_num_attacks(int value) { num_attacks_ = value; set_changed(true); }
+	void set_attack_weight(double value) { attack_weight_ = value; set_changed(true); }
+	void set_defense_weight(double value) { defense_weight_ = value; set_changed(true); }
+	void set_specials(config value) { specials_ = value; set_changed(true); }
 
 	// In action/attack.cpp
 	std::pair<int, bool> combat_ability(const std::string& ability, int abil_value = 0, bool backstab_pos = false) const;
@@ -151,6 +151,14 @@ class attack_type : public std::enable_shared_from_this<attack_type>
 	specials_context_t specials_context_for_listing(bool attacking = true) const {
 		return specials_context_t(*this, attacking);
 	}
+	void set_changed(bool value)
+	{
+		changed_ = value;
+	}
+	bool get_changed() const
+	{
+		return changed_;
+	}
 private:
 
 	t_string description_;
@@ -168,6 +176,7 @@ class attack_type : public std::enable_shared_from_this<attack_type>
 	int movement_used_;
 	int parry_;
 	config specials_;
+	bool changed_;
 };
 
 using attack_list = std::vector<attack_ptr>;