fix(diff): use explicit change-type if possible

That way, we do not have to figure the change type out by
examining the diff object.

It's implemented in a way that should yield more desireable results
as we keep the change-type that git is providing us with.

Fixes #493

Author: Byron

Diff for: doc/source/changes.rst

@@ -2,6 +2,12 @@
 Changelog
 =========
 
+2.0.8 - Bugfixes
+================
+
+* `DiffIndex.iter_change_type(...)` produces better results when diffing
+  an index against the working tree.
+
 2.0.7 - New Features
 ====================
 

Diff for: git/diff.py

@@ -183,7 +183,9 @@ def iter_change_type(self, change_type):
             raise ValueError("Invalid change type: %s" % change_type)
 
         for diff in self:
-            if change_type == "A" and diff.new_file:
+            if diff.change_type == change_type:
+                yield diff
+            elif change_type == "A" and diff.new_file:
                 yield diff
             elif change_type == "D" and diff.deleted_file:
                 yield diff
@@ -247,11 +249,12 @@ class Diff(object):
     NULL_BIN_SHA = b"\0" * 20
 
     __slots__ = ("a_blob", "b_blob", "a_mode", "b_mode", "a_rawpath", "b_rawpath",
-                 "new_file", "deleted_file", "raw_rename_from", "raw_rename_to", "diff")
+                 "new_file", "deleted_file", "raw_rename_from", "raw_rename_to",
+                 "diff", "change_type")
 
     def __init__(self, repo, a_rawpath, b_rawpath, a_blob_id, b_blob_id, a_mode,
                  b_mode, new_file, deleted_file, raw_rename_from,
-                 raw_rename_to, diff):
+                 raw_rename_to, diff, change_type):
 
         self.a_mode = a_mode
         self.b_mode = b_mode
@@ -286,6 +289,7 @@ def __init__(self, repo, a_rawpath, b_rawpath, a_blob_id, b_blob_id, a_mode,
         self.raw_rename_to = raw_rename_to or None
 
         self.diff = diff
+        self.change_type = change_type
 
     def __eq__(self, other):
         for name in self.__slots__:
@@ -435,7 +439,7 @@ def _index_from_patch_format(cls, repo, stream):
                               new_file, deleted_file,
                               rename_from,
                               rename_to,
-                              None))
+                              None, None))
 
             previous_header = header
         # end for each header we parse
@@ -483,7 +487,7 @@ def _index_from_raw_format(cls, repo, stream):
             # END add/remove handling
 
             diff = Diff(repo, a_path, b_path, a_blob_id, b_blob_id, old_mode, new_mode,
-                        new_file, deleted_file, rename_from, rename_to, '')
+                        new_file, deleted_file, rename_from, rename_to, '', change_type)
             index.append(diff)
         # END for each line
 

Diff for: git/test/fixtures/diff_abbrev-40_full-index_M_raw_no-color

@@ -0,0 +1 @@
+:100644 100644 739bc65220ad90e9ebfa2d6af1723b97555569a4 0000000000000000000000000000000000000000 M	README.md

Diff for: git/test/test_diff.py

@@ -104,6 +104,15 @@ def test_diff_with_rename(self):
         assert diff.rename_to == 'that'
         assert len(list(diffs.iter_change_type('R'))) == 1
 
+    def test_diff_of_modified_files_not_added_to_the_index(self):
+        output = StringProcessAdapter(fixture('diff_abbrev-40_full-index_M_raw_no-color'))
+        diffs = Diff._index_from_raw_format(self.rorepo, output.stdout)
+        
+        assert len(diffs) == 1, 'one modification'
+        assert len(list(diffs.iter_change_type('M'))) == 1, 'one modification'
+        assert diffs[0].change_type == 'M'
+        assert diffs[0].b_blob is None
+        
     def test_binary_diff(self):
         for method, file_name in ((Diff._index_from_patch_format, 'diff_patch_binary'),
                                   (Diff._index_from_raw_format, 'diff_raw_binary')):