Clean up character mutation gain a bit

Reduce duplicated code.
I presume there's a reason that set_mutations didn't just call
set_mutation, and the only difference is that it doesn't do the cache
updates till the end, so preserve that behaviour.

src/character.h

@@ -987,6 +987,13 @@ class Character : public Creature, public visitable
         /** Add or removes a mutation on the player, but does not trigger mutation loss/gain effects. */
         void set_mutations( const std::vector<trait_id> &traits );
         void set_mutation( const trait_id & );
+    protected:
+        // Set a mutation, but don't do any of the necessary updates
+        // Only call this from one of the above two functions
+        void set_mutation_unsafe( const trait_id & );
+    public:
+        // Do the mutation updates necessary when adding a mutation (nonspecific cache updates)
+        void do_mutation_updates();
         void unset_mutation( const trait_id & );
         /**Unset switched mutation and set target mutation instead*/
         void switch_mutations( const trait_id &switched, const trait_id &target, bool start_powered );

src/mutation.cpp

@@ -158,27 +158,7 @@ void Character::toggle_trait( const trait_id &trait_ )
     }
 }
 
-void Character::set_mutations( const std::vector<trait_id> &traits )
-{
-    for( const trait_id &trait : traits ) {
-        const auto iter = my_mutations.find( trait );
-        if( iter != my_mutations.end() ) {
-            continue;
-        }
-        my_mutations.emplace( trait, trait_data{} );
-        cached_mutations.push_back( &trait.obj() );
-        mutation_effect( trait, false );
-    }
-    recalc_sight_limits();
-    calc_encumbrance();
-
-    // If the stamina is higher than the max (Languorous), set it back to max
-    if( get_stamina() > get_stamina_max() ) {
-        set_stamina( get_stamina_max() );
-    }
-}
-
-void Character::set_mutation( const trait_id &trait )
+void Character::set_mutation_unsafe( const trait_id &trait )
 {
     const auto iter = my_mutations.find( trait );
     if( iter != my_mutations.end() ) {
@@ -187,6 +167,10 @@ void Character::set_mutation( const trait_id &trait )
     my_mutations.emplace( trait, trait_data{} );
     cached_mutations.push_back( &trait.obj() );
     mutation_effect( trait, false );
+}
+
+void Character::do_mutation_updates()
+{
     recalc_sight_limits();
     calc_encumbrance();
 
@@ -196,6 +180,20 @@ void Character::set_mutation( const trait_id &trait )
     }
 }
 
+void Character::set_mutations( const std::vector<trait_id> &traits )
+{
+    for( const trait_id &trait : traits ) {
+        set_mutation_unsafe( trait );
+    }
+    do_mutation_updates();
+}
+
+void Character::set_mutation( const trait_id &trait )
+{
+    set_mutation_unsafe( trait );
+    do_mutation_updates();
+}
+
 void Character::unset_mutation( const trait_id &trait_ )
 {
     // Take copy of argument because it might be a reference into a container