rebase: abort if the user tries to rebase the working copy
I think it's more correct to treat hg rebase -r 'wdirrev()' -d foo
as hg co -m foo, but I'm instead making it error out. That's partly
because it's probably what the user wanted (in the case I heard from a
user, they had done hg rebase -s f where f resolved to wdir())
and partly because I don't want to think about more complicated cases
where the user specifies the working copy together with other commits.
Differential Revision: https://phab.mercurial-scm.org/D8057