Improve add_facts performance

When making multiple add_facts calls to add facts to an already
present and large set of target facts performance declines
as more are added due to the initialization and copying of
the large hashes each time.

This change merges the added facts in place to the existing
facts, which provides a slight performance improvement.

!feature

* **Minor add_facts optimization**

  Deep merge new facts into existing target fact hash, instead
  of creating and copying a new hash each time.

lib/bolt/inventory/target.rb

@@ -98,7 +98,7 @@ def facts
 
       def add_facts(new_facts = {})
         validate_fact_names(new_facts)
-        @facts = Bolt::Util.deep_merge(@facts, new_facts)
+        Bolt::Util.deep_merge!(@facts, new_facts)
       end
 
       def features

lib/bolt/util.rb

@@ -212,6 +212,17 @@ def deep_merge(hash1, hash2)
         hash1.merge(hash2, &recursive_merge)
       end
 
+      def deep_merge!(hash1, hash2)
+        recursive_merge = proc do |_key, h1, h2|
+          if h1.is_a?(Hash) && h2.is_a?(Hash)
+            h1.merge!(h2, &recursive_merge)
+          else
+            h2
+          end
+        end
+        hash1.merge!(hash2, &recursive_merge)
+      end
+
       # Accepts a Data object and returns a copy with all hash keys
       # modified by block. use &:to_s to stringify keys or &:to_sym to symbolize them
       def walk_keys(data, &block)