Fix broken logic in merge command (`--push` arguments)

The logic used to determine the `update` flag in `rmerge` is currently broken because:
- fast-forwards and merges in the parent repo are not accounted for
- submodule update status can override this

Concrete example: the [SCC-merge](http://hudson.openmicroscopy.org.uk/view/Mgmt/job/SCC-merge/) self-merge step cannot push to `merge/master/latest`.This PR should fix this logic.

Since this is a major bug, I suggest once this PR and #73 get reviewed and merged we tag 0.3.3 and upload to pypi.