svn log messages containing utf-8 (such as r773) don't get displayed
right in RecentChanges. The problem is ikiwiki runs svn log in locale C,
which makes it spit out eacaped charcters for utf-8 chars. If it's run in
locale en_US.UTF-8, it would be ok, but that would require the system
have that locale.

Seems that the right fix for this is to use svn log --xml, which is
always utf-8 and come up with a parser for that. Also fixes the spoofing
issue in [[security]].

[[bugs/done]]
