Suppress base64'd content in non-english "Content changed from..." message

The previous check only functioned if the user's language was English,
as the call to ->SetSubValues returns a localized string.  Rather than
special-casing on the wording, instead override the message based on the
return code.

Fixes I#24665.
Never return 'Changed (base64) to (base64)' as a message

Similar to d24effb1, but even more so -- the end-user cares not for
giant base64 blobs.
Merge branch '4.2/attribute-change-msg' into 4.2-trunk