diff --git a/git/objects/tree.py b/git/objects/tree.py
index e7d579d0e..4f853f92a 100644
--- a/git/objects/tree.py
+++ b/git/objects/tree.py
@@ -35,33 +35,42 @@ def git_cmp(t1, t2):
     if min_cmp:
         return min_cmp
 
-    return len_b - len_a
+    return len_a - len_b
 
-if PY3:
-    # taken from https://wiki.python.org/moin/HowTo/Sorting#The_Old_Way_Using_the_cmp_Parameter
-    class CmpToKey(object):
-        __slots__ = 'obj'
 
-        def __init__(self, obj, *args):
-            self.obj = obj
+def merge_sort(a, cmp):
+    if len(a) < 2:
+        return
 
-        def __lt__(self, other):
-            return git_cmp(self.obj, other.obj) < 0
+    mid = len(a) // 2
+    lefthalf = a[:mid]
+    righthalf = a[mid:]
 
-        def __gt__(self, other):
-            return git_cmp(self.obj, other.obj) > 0
+    merge_sort(lefthalf, cmp)
+    merge_sort(righthalf, cmp)
 
-        def __eq__(self, other):
-            return git_cmp(self.obj, other.obj) == 0
+    i = 0
+    j = 0
+    k = 0
 
-        def __le__(self, other):
-            return git_cmp(self.obj, other.obj) <= 0
+    while i < len(lefthalf) and j < len(righthalf):
+        if cmp(lefthalf[i], righthalf[j]) <= 0:
+            a[k] = lefthalf[i]
+            i = i + 1
+        else:
+            a[k] = righthalf[j]
+            j = j + 1
+        k = k + 1
 
-        def __ge__(self, other):
-            return git_cmp(self.obj, other.obj) >= 0
+    while i < len(lefthalf):
+        a[k] = lefthalf[i]
+        i = i + 1
+        k = k + 1
 
-        def __ne__(self, other):
-            return git_cmp(self.obj, other.obj) != 0
+    while j < len(righthalf):
+        a[k] = righthalf[j]
+        j = j + 1
+        k = k + 1
 
 
 class TreeModifier(object):
@@ -90,10 +99,7 @@ def set_done(self):
         It may be called several times, but be aware that each call will cause
         a sort operation
         :return self:"""
-        if PY3:
-            self._cache.sort(key=CmpToKey)
-        else:
-            self._cache.sort(cmp=git_cmp)
+        merge_sort(self._cache, git_cmp)
         return self
     #} END interface
 
diff --git a/git/test/test_tree.py b/git/test/test_tree.py
index 5e94e70bb..a7cdd9c6b 100644
--- a/git/test/test_tree.py
+++ b/git/test/test_tree.py
@@ -77,30 +77,10 @@ def test_serializable(self):
             # del again, its fine
             del(mod[invalid_name])
 
-            # SPECIAL ORDERING !
-            # Add items which should sort differently from standard alum sort order
-            mod.add(hexsha, Tree.blob_id << 12, 'fild')
-            mod.add(hexsha, Tree.blob_id << 12, 'file')
-            mod.add(hexsha, Tree.blob_id << 12, 'file.second')
-            mod.add(hexsha, Tree.blob_id << 12, 'filf')
-
-            def names_in_mod_cache():
-                return [t[2] for t in mod._cache]
-
-            def chunk_from(a, name, size):
-                index = a.index(name)
-                return a[index:index + size]
-
-            assert chunk_from(names_in_mod_cache(), 'fild', 4) == ['fild', 'file', 'file.second',
-                                                                   'filf']
-
             # have added one item, we are done
             mod.set_done()
             mod.set_done()      # multiple times are okay
 
-            assert chunk_from(names_in_mod_cache(), 'fild', 4) == ['fild', 'file.second', 'file',
-                                                                   'filf']
-
             # serialize, its different now
             stream = BytesIO()
             testtree._serialize(stream)
@@ -114,6 +94,41 @@ def chunk_from(a, name, size):
             assert invalid_name not in testtree
         # END for each item in tree
 
+    def test_tree_modifier_ordering(self):
+        hexsha = '6c1faef799095f3990e9970bc2cb10aa0221cf9c'
+        roottree = self.rorepo.tree(hexsha)
+        blob_mode = Tree.blob_id << 12
+        tree_mode = Tree.tree_id << 12
+
+        files_in_desired_order = [
+            (blob_mode, '_.htaccess'),
+            (tree_mode, 'fileadmin'),
+            (blob_mode, 'index.php'),
+            (tree_mode, 'typo3'),
+            (tree_mode, 'typo3_src-6.2.12'),
+            (tree_mode, 'typo3_src'),
+            (blob_mode, 'typo3cms'),
+            (tree_mode, 'typo3conf'),
+            (tree_mode, 'uploads'),
+        ]
+        mod = roottree.cache
+        for (file_mode, file_name) in files_in_desired_order:
+            mod.add(hexsha, file_mode, file_name)
+        # end for each file
+
+        def file_names_in_order():
+            return [t[1] for t in files_in_desired_order]
+
+        def names_in_mod_cache():
+            a = [t[2] for t in mod._cache]
+            here = file_names_in_order()
+            return [e for e in a if e in here]
+
+        assert names_in_mod_cache() == file_names_in_order(), 'add() keeps order'
+
+        mod.set_done()
+        assert names_in_mod_cache() == file_names_in_order(), 'set_done() performs git-sorting'
+
     def test_traverse(self):
         root = self.rorepo.tree('0.1.6')
         num_recursive = 0