Use print_modifier, not signed_value, when dealing with percentages.

Fixes bug #21577.

changelog

@@ -17,6 +17,8 @@ Version 1.11.10+dev:
    * Changed: Updated valgrind suppression file.
    * Labels are now removed when shroud/fog is removed, rather than waiting
      for a new turn (bug #21434).
+   * Percent signs show when describing traits that increase damage or attacks
+     by a percentage (bug #21577).
 
 Version 1.11.10:
  * Add-ons client:

players_changelog

@@ -18,6 +18,8 @@ Version 1.11.10+dev:
    * Units can no longer be moved in linger mode (bug #21450).
    * Labels are now removed when shroud/fog is removed, rather than waiting
      for a new turn (bug #21434).
+   * Percent signs show when describing traits that increase damage or attacks
+     by a percentage (bug #21577).
 
 
 Version 1.11.10:

src/unit_types.cpp

@@ -245,7 +245,7 @@ bool attack_type::apply_modification(const config& cfg,std::string* description)
 
 		if(description != NULL) {
 			int inc_damage = lexical_cast<int>(increase_damage);
-			desc << utils::signed_value(inc_damage) << " "
+			desc << utils::print_modifier(increase_damage) << " "
 				 << _n("damage","damage", inc_damage);
 		}
 	}
@@ -256,7 +256,7 @@ bool attack_type::apply_modification(const config& cfg,std::string* description)
 
 		if(description != NULL) {
 			int inc_attacks = lexical_cast<int>(increase_attacks);
-			desc << utils::signed_value(inc_attacks) << " "
+			desc << utils::print_modifier(increase_attacks) << " "
 				 << _n("strike", "strikes", inc_attacks);
 		}
 	}
@@ -326,13 +326,13 @@ bool attack_type::describe_modification(const config& cfg,std::string* descripti
 
 		if(increase_damage.empty() == false) {
 			int inc_damage = lexical_cast<int>(increase_damage);
-			desc << utils::signed_value(inc_damage) << " "
+			desc << utils::print_modifier(increase_damage) << " "
 				 << _n("damage","damage", inc_damage);
 		}
 
 		if(increase_attacks.empty() == false) {
 			int inc_attacks = lexical_cast<int>(increase_attacks);
-			desc << utils::signed_value(inc_attacks) << " "
+			desc << utils::print_modifier(increase_attacks) << " "
 				 << _n("strike", "strikes", inc_attacks);
 		}