patman: Correct pylint errors

Fix pylint errors that can be fixed and mask those that seem to be
incorrect.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/patman/commit.py

@@ -31,7 +31,7 @@ class Commit:
     """
     def __init__(self, hash):
         self.hash = hash
-        self.subject = None
+        self.subject = ''
         self.tags = []
         self.changes = {}
         self.cc_list = []