Truncate null revision comment for whole multibyte characters

Authored by Umherirrender.

Description

Truncate null revision comment for whole multibyte characters

Added a truncate for multibyte characters to Revision::newNullRevision.
This avoids broken characters for long comment, mostly the comment for
move or protection in the history.
The whole comment is still visible on Special:Log.

Bug: T85700
Change-Id: Ie0863261484ee99006407c12c3a920ea476925e3

Details