Update-submodules PR update

This PR follows the work carried on in #93. In the case of the `scc update-submodules` command, if a submodules PR is updated by the push to Github, no comment is added but the body of the PR is replaced using the new commit content instead.

This PR also reviews/adds new integration tests for the Merge and UpdateSubmodules commands. 

To test the PR, check the snoopys-sandbox PR generated by the `TestUpdateSubmodules.testPushUpdatePR` test does not create an additional comment and that the body is accurate in the [SCC-self-merge](http://hudson.openmicroscopy.org.uk/view/Mgmt/job/SCC-self-merge/) job.