destutil: allow to specify an explicit source for the merge We can now specify from where the merge is performed. The experimental revset is updated to take revisions as argument, allowing to test the feature. This will become very useful for pick the 'rebase' default destination. For this reason, we also exclude all descendants from the rebased set from the candidate destinations. This descendants exclusion was not necessary for merge as default destination would not be picked from anything else than a head. I'm not super excited with the current error messages, but I would prefer to delay an overall messages rework once 'hg rebase' is done getting a default destination aligned with 'hg merge'.