octopus merges are not supported -- add an xfail test?

[fingolfin]

Octopus merges on the git side will cause a failure when trying to push them. They are of course rather obscure, but we should make sure that we at least do not choke at them. So, there should be a test for that (either a passing test for the right error message to be printed; or an xfailing one attempting to push an octopus merge to git).

If somebody wants to work on this, one idea on how to implement it (due to Johannes Schindelin) is briefly described here: https://github.com/msysgit/msysgit/wiki/Guide-to-git-remote-hg