The bzr plugin does not seem to define the rcs_diff subroutine.
I got the follow error after enabling recentchangesdiff:

"Undefined subroutine &IkiWiki::Plugin::bzr::rcs_diff called at /usr/share/perl5/IkiWiki.pm line 1590."

Grepping to verify absence of rcs_diff:

    $ grep rcs_diff /usr/share/perl5/IkiWiki/Plugin/{git,bzr}.pm
    /usr/share/perl5/IkiWiki/Plugin/git.pm:	hook(type => "rcs", id => "rcs_diff", call => \&rcs_diff);
    /usr/share/perl5/IkiWiki/Plugin/git.pm:sub rcs_diff ($) {
    /usr/share/perl5/IkiWiki/Plugin/bzr.pm:	hook(type => "rcs", id => "rcs_diff", call => \&rcs_diff);