rewriteutil: adapt "cannot %s while merging" to work with "change branch of" `rewriteutil.precheck()` creates error messages by inserting a given verb into a sentence. The `hg branch -r` command passes in "change branch of" as the verb. That doesn't work well with "cannot %s while merging" (making it "cannot change branch of while merging"). Let's insert a "changeset" there to make it work better. Building sentences like this seems obviously bad for i18n, but fixing that is out of scope for this series, IMO. Differential Revision: https://phab.mercurial-scm.org/D10530

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "phabricator.uri": "https://phab.mercurial-scm.org/",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}