This is intended to help automate the process of pull changes down from
ruby-build upstream. It depends on customized rbenv-tags refspec for
namespacing ruby-build's tags (that way ruby-builds tags don't pollute
node-build's tags). It re-uses a staging branch where ruby-build's tags
will be merged into. It finds only the very "next" un-merged ruby-build
tag; diffs against that and merges it. Then removes the ruby-build defs
and known scripts that don't apply to node-build. Should leave the
working copy in a MERGING state.
This is very much WIP: