When an attempt to update to a hidden changeset fails because the
working copy is dirty, you may get a message like this:
updating to a hidden changeset 343f6de32686 (hidden revision '343f6de32686' was rewritten as: 4ab941244072) abort: conflicting changes (commit or update --clean to discard changes)
It's easy to miss the real error here. This patch moves the warning
about the hidden changeset to after the update has happened. It
changes the verb tense accordingly (and drops the "a" that I think it
sounds better without). Of course, this means that the commit isn't
actually hidden anymore when the message is printed. I think that's
fine.