utils: checkstyle.py: Derive Amendment from Commit

The Amendment commit class is derived from the StagedChanges class
(which in turn derives from the Commit base class), however there is no
code sharing between Amendment and StagedChanges other than the call to
initalise through the base Commit class.

Refactor the inheritance to make an Amendment derive directly from
Commit.

Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Tested-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

utils/checkstyle.py

@@ -269,9 +269,9 @@ class StagedChanges(Commit):
         return parse_diff(diff.splitlines(True))
 
 
-class Amendment(StagedChanges):
+class Amendment(Commit):
     def __init__(self):
-        StagedChanges.__init__(self)
+        Commit.__init__(self, '')
 
     def _parse(self):
         # Create a title using HEAD commit