ikiwiki/ todo/ multiple simultaneous rcs

One useful item would be support for multiple RCS systems simultaneously.

Example:

I'm not sure how the mechanics would work out for choosing between what to refs and managing branch self-references (though doesn't ikiwiki do something like this:

Ikiwiki checks: 1/b/a/j, then 1/a/j and succeeds...

The new git subprojects might work, but I've had trouble making them work sanely in the past...