This is fixed in 1fe6f365df2e, now in crew and main.

I have heard that OpenJDK uses gateway (push-only) repos which prevent named
branches from being pushed to the public repo for just this reason.

This is similar, but not identical, to Issue736.

As soon as I have a branch with a head on it, the plain "hg merge" command does
not work anymore after pulling from the master repository. The reason is obvious
(3 heads), but this is surprising to my hg-novice coworkers.

Also, a plain "hg merge" in a repo with 2 heads in different named branches
unexpectedly succeeds, and probably does not do what the user expected.

I think that "hg merge" should only count heads that are on the same named
branch as the current checkout.