Fix type error (startswith expects bytes)

Author: johnwalker

git/objects/commit.py

@@ -445,7 +445,7 @@ def _deserialize(self, stream):
         next_line = readline()
         while next_line.startswith(b'mergetag '):
             next_line = readline()
-            while next_line.startswith(' '):
+            while next_line.startswith(b' '):
                 next_line = readline()
         # end skip mergetags