Split diff line by '\t' for metadata and path

This protects against `.split(None)` which uses consecutive whitespace
as a separator to overlook paths where a single space is the filename.
For example, in this diff line:

line = ':100644 000000 e69de29
0000000000000000000000000000000000000000 D       '

The deleted file is a file named ' ' (just one space).  It's entirely
possible to commit this, remove, and to produce the following output
from `git diff`:

git diff --name-status <SHA1> <SHA2>
D
M       path/to/another/file.py
...

This would cause the initial `.split(None, 5)` to fail as it will count
all consecutive whitespace as a separator, disregarding the ' ' (single
space)  filename.

git/diff.py

@@ -365,7 +365,8 @@ def _index_from_raw_format(cls, repo, stream):
             if not line.startswith(":"):
                 continue
             # END its not a valid diff line
-            old_mode, new_mode, a_blob_id, b_blob_id, change_type, path = line[1:].split(None, 5)
+            meta, _, path = line[1:].partition('\t')
+            old_mode, new_mode, a_blob_id, b_blob_id, change_type = meta.split(None, 4)
             path = path.strip()
             a_path = path
             b_path = path