error on pushing a diverged branch to append_only target

[Tim Penhey]

I created a very small bzr plugin command to allow me to tweak LP branches to be append_only like this:

class cmd_append_only(Command):
    takes_args = ['branch']
    def run(self, branch):
        """See `Command`."""
        b = Branch.open(branch)
        b.lock_write()
        b.set_append_revisions_only(True)
        b.unlock()

And in testing it I did the following:

I took an existing project I had, like unity:
  bzr cbranch trunk test
  cd test
  bzr push test
  bzr append-only :push (failed but not this bug)
  bzr append-only lp:~thumper/unity/test
  bzr uncommit
  bzr revert
  bzr push (silently succeeded, again, not this bug)
  bzr commit --unchanged
  bzr push
  Using saved push location: lp:~thumper/unity/test
  bzr: ERROR: These branches have diverged. See "bzr help diverged-branches" for more information.
  -- this is fine
  bzr push --overwrite
  Using saved push location: lp:~thumper/unity/test
bzr: ERROR: Server sent an unexpected error: ('error', 'RevisionNotPresent', 'Revision {tarmac-20120212192717-9jmrqx6we7yd8bl7} not present in "Graph(StackedParentsProvider(bzrlib.repository._LazyListJoin(([CachingParentsProvider(CallableToParentsProviderAdapter(<bound method CHKInventoryRepository._get_parent_map_no_fallbacks of CHKInventoryRepository(\'chroot-95047632:///~thumper/unity/test/.bzr/repository/\')>))], []))))".')

Not so good.