Avoid a potential TypeError in util.ArFile.saveTo

If no dir is passed to util.ArFile.saveTo, dir is set to os.getcwd(),
which returns a str. Since self.name is a bytes, the subsequent
os.path.join(dir, self.name) results in a TypeError.
To fix this, use os.getcwdb(), which returns a bytes instead of a
str.

osc/util/ar.py

@@ -72,7 +72,7 @@ class ArFile(BytesIO):
         and permissions.
         """
         if not dir:
-            dir = os.getcwd()
+            dir = os.getcwdb()
         fn = os.path.join(dir, self.name)
         with open(fn, 'wb') as f:
             f.write(self.getvalue())