phases: rewrite "immutable changeset" to "public changeset" The phrase "cannot edit immutable changeset" is kind of tautological. Of course unchangeable things can't be changed. We instead mention "public" and provide a hint so that we can point to the actual problem. Even in cases where some operation other than edition cannot be performed, "public" gives the root cause that results in the "immutable" effect. There is a precedent for saying "public" instead of "immutable", for example, in `hg commit --amend`.