It'd be nice if githelper could be extended to block such merges, i.e. if a commit moves the merge base between any two main (X.Y-style) branches, it's blocked.
Well, we'd have to add a definition of a "main branch" to the code then. Right now it's mostly generic. And if it should work with commit and not just push, it also needs to consider local branches tracking a "main branch" at origin.