added docstring for unified_diff implementation

dulwich/patch.py

@@ -101,6 +101,11 @@ def _format_range_unified(start, stop):
 
 def unified_diff(a, b, fromfile='', tofile='', fromfiledate='',
                  tofiledate='', n=3, lineterm='\n'):
+    """difflib.unified_diff that can detect "No newline at end of file" as
+    original "git diff" does.
+
+    Based on the same function in Python2.7 difflib.py
+    """
     started = False
     for group in SequenceMatcher(None, a, b).get_grouped_opcodes(n):
         if not started: