fix(diff): mode-assertions now deal with 0

If the file was not present, the mode seen in a diff can be legally '0',
which previously caused an assertion to fail for no good reason.
Now the assertion tests for None instead.

Closes #323

2 files changed, 34 insertions, 2 deletions

diff --git a/git/diff.py b/git/diff.py
index dc53f3f7..9059091e 100644
--- a/git/diff.py
+++ b/git/diff.py
@@ -223,12 +223,12 @@ class Diff(object):
         if a_blob_id is None:
             self.a_blob = None
         else:
-            assert self.a_mode
+            assert self.a_mode is not None
             self.a_blob = Blob(repo, hex_to_bin(a_blob_id), mode=self.a_mode, path=a_path)
         if b_blob_id is None:
             self.b_blob = None
         else:
-            assert self.b_mode
+            assert self.b_mode is not None
             self.b_blob = Blob(repo, hex_to_bin(b_blob_id), mode=self.b_mode, path=b_path)
 
         self.new_file = new_file
diff --git a/git/test/test_diff.py b/git/test/test_diff.py
index f2ce1447..53bb65db 100644
--- a/git/test/test_diff.py
+++ b/git/test/test_diff.py
@@ -4,6 +4,7 @@
 #
 # This module is part of GitPython and is released under
 # the BSD License: http://www.opensource.org/licenses/bsd-license.php
+import os
 
 from git.test.lib import (
     TestBase,
@@ -14,7 +15,11 @@ from git.test.lib import (
 
 )
 
+from gitdb.test.lib import with_rw_directory
+
 from git import (
+    Repo,
+    GitCommandError,
     Diff,
     DiffIndex
 )
@@ -37,6 +42,33 @@ class TestDiff(TestBase):
         # END for each diff
         return diffs
 
+    @with_rw_directory
+    def test_diff_with_staged_file(self, rw_dir):
+        # SETUP INDEX WITH MULTIPLE STAGES
+        r = Repo.init(rw_dir)
+        fp = os.path.join(rw_dir, 'hello.txt')
+        with open(fp, 'w') as fs:
+            fs.write("hello world")
+        r.git.add(fp)
+        r.git.commit(message="init")
+
+        with open(fp, 'w') as fs:
+            fs.write("Hola Mundo")
+        r.git.commit(all=True, message="change on master")
+
+        r.git.checkout('HEAD~1', b='topic')
+        with open(fp, 'w') as fs:
+            fs.write("Hallo Welt")
+        r.git.commit(all=True, message="change on topic branch")
+
+        # there must be a merge-conflict
+        self.failUnlessRaises(GitCommandError, r.git.cherry_pick, 'master')
+
+        # Now do the actual testing - this should just work
+        assert len(r.index.diff(None)) == 2
+
+        assert len(r.index.diff(None, create_patch=True)) == 0, "This should work, but doesn't right now ... it's OK"
+
     def test_list_from_string_new_mode(self):
         output = StringProcessAdapter(fixture('diff_new_mode'))
         diffs = Diff._index_from_patch_format(self.rorepo, output.stdout)