Handle empty blobs correctly

If a file was added or deleted, GitPython will indicate the blob as None. In the current case, we would always read the file from disk so the file contents would be same as the Working Tree. This commit modifies that handling to handle the Working Tree case separately, and the None blob case separately.

Tested on the both the nbdime and Git lab extensions.

nbdime/gitfiles.py

@@ -1,8 +1,8 @@
 #!/usr/bin/env python
 # -*- coding:utf-8 -*-
 
-import os
 import io
+import os
 from collections import deque
 
 from six import string_types
@@ -104,11 +104,12 @@ def _get_diff_entry_stream(path, blob, ref_name, repo_dir):
     """Get a stream to the notebook, for a given diff entry's path and blob
 
     Returns None if path is not a Notebook file, and EXPLICIT_MISSING_FILE
-    if path is missing."""
+    if path is missing, or the blob is None.
+    """
     if path:
         if not path.endswith('.ipynb'):
             return None
-        if blob is None:
+        if ref_name is GitRefWorkingTree:
             # Diffing against working copy, use file on disk!
             with pushd(repo_dir):
                 # Ensure we filter if appropriate:
@@ -122,6 +123,10 @@ def _get_diff_entry_stream(path, blob, ref_name, repo_dir):
                     return io.open(path, encoding='utf-8')
                 except IOError:
                     return EXPLICIT_MISSING_FILE
+        elif blob is None:
+            # GitPython uses a None blob to indicate if a file was deleted or added.
+            # https://gitpython.readthedocs.io/en/stable/reference.html#git.diff.Diff
+            return EXPLICIT_MISSING_FILE
         else:
             # There were strange issues with passing blob data_streams around,
             # so we solve this by reading into a StringIO buffer.