performance optimisation in hash merge logic

rewrite deepcopy in util.merge_hash and just iterate
on an inventory with 500 groups and 800 hosts this brings back the
inventory initialisation from 13s to 3s (with hash_behaviour=merge)

lib/ansible/utils/__init__.py

@@ -558,10 +558,11 @@ def merge_hash(a, b):
     ''' recursively merges hash b into a
     keys from b take precedence over keys from a '''
 
-    result = copy.deepcopy(a)
+    result = {}
 
+    for dicts in a, b:
         # next, iterate over b keys and values
-    for k, v in b.iteritems():
+        for k, v in dicts.iteritems():
             # if there's already such key in a
             # and that key contains dict
             if k in result and isinstance(result[k], dict):