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);

> I've added the minimal stub needed to avoid the crash, but for
> recentchangesdiff to work, someone needs to implement `rcs_diff` for bzr.
> This should be trivial if you know and use bzr. The function
> is passed as a parameter the revno of interest and just needs
> to ask bzr for the diff between that and the previous version. --[[Joey]] 

>> I'll see if I can make a patch. The bzr command to get the revision would
>> look like this: bzr diff -r revno:$PREV:/path/to/src..revno:$REVNO:/path/to/src
>> (where $PREV would be $REVNO minus one). --liw

>> Sorry, that was not entirely correct, for some reason. I'll add a patch below that
>> seems to work. I am unfortunately not ready to set up a git repository that you
>> can pull from. --liw

    diff --git a/IkiWiki/Plugin/.bzr.pm.swp b/IkiWiki/Plugin/.bzr.pm.swp
    new file mode 100644
    index 0000000..712120c
    Binary files /dev/null and b/IkiWiki/Plugin/.bzr.pm.swp differ
    diff --git a/IkiWiki/Plugin/bzr.pm b/IkiWiki/Plugin/bzr.pm
    index 783623d..f1d5854 100644
    --- a/IkiWiki/Plugin/bzr.pm
    +++ b/IkiWiki/Plugin/bzr.pm
    @@ -256,7 +256,25 @@ sub rcs_recentchanges ($) {
     }
   
     sub rcs_diff ($) {
    -   # TODO
    +        my $taintedrev=shift;
    +   my ($rev) = $taintedrev =~ /^(\d+(\.\d+)*)$/; # untaint
    +           print STDERR "taintedrev: $taintedrev\nrev: $rev\n";
    +
    +   my $prevspec = "before:" . $rev;
    +   my $revspec = "revno:" . $rev;
    +        my @cmdline = ("bzr", "diff", "--old", $config{srcdir},
    +                  "--new", $config{srcdir},
    +                  "-r", $prevspec . ".." . $revspec);
    +   print STDERR "cmdline: @cmdline\n";
    +        open (my $out, "@cmdline |");
    +
    +   my @lines = <$out>;
    +        if (wantarray) {
    +                return @lines;
    +        }
    +        else {
    +                return join("", @lines);
    +        }
     }
   
     sub rcs_getctime ($) {