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.