Get dirtymark merge working:
* Todo: describe a bug that seems like coming from perl
* FakeBigFloat: disable native floating math until this bug is analysed
* Recentfile::aggregate now with option "force"
* Recentfile::aggregate now calls an empty update() on the first recentfile
* Recentfile::aggregate now DTRT when the other file has a newer dirtymark
* Recentfile::_sparse_clone now does not blindly copy the dirtymark anymore
* Recentfile::update can now be called without argument to get "truncate"
* Recentfile::merge now DTRT when it has no dirtymark yet
* 02-operation.t: add tests to see dirtymark in action
* few amendments to 02-operation.t needed because aggregate now calls update