histedit: byteify the help for the multifold action

While there's some allowance for str in `_()`, it's commented to be for "goofy
unicode docstrings in test", so no idea how well that works, but it should at
least come back as bytes.  With HGPLAIN, however, the str isn't touched and is
returned as-is, so this seems like a real bug.

hgext/histedit.py

@@ -996,7 +996,7 b' class base(histeditaction):'
 @action(
     [b'_multifold'],
     _(
-        """fold subclass used for when multiple folds happen in a row
+        b"""fold subclass used for when multiple folds happen in a row
 
     We only want to fire the editor for the folded message once when
     (say) four changes are folded down into a single change. This is