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

@@ -558,18 +558,19 @@ def merge_hash(a, b):
     ''' recursively merges hash b into a
     keys from b take precedence over keys from a '''
 
-    result = copy.deepcopy(a)
-
-    # next, iterate over b keys and values
-    for k, v in b.iteritems():
-        # if there's already such key in a
-        # and that key contains dict
-        if k in result and isinstance(result[k], dict):
-            # merge those dicts recursively
-            result[k] = merge_hash(a[k], v)
-        else:
-            # otherwise, just copy a value from b to a
-            result[k] = v
+    result = {}
+
+    for dicts in a, b:
+        # next, iterate over b keys and values
+        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):
+                # merge those dicts recursively
+                result[k] = merge_hash(a[k], v)
+            else:
+                # otherwise, just copy a value from b to a
+                result[k] = v
 
     return result