Ensure that repo changes can be merged

[jgadsden]

Describe what problem your feature request solves:
Threat Dragon needs to be able to handle the case where the model has been changed on the repo while the model is also being edited within Threat Dragon - in which case potentially a merge to the repo is required to resolve any conflicts

Describe the solution you'd like:
Ensure that Threat Dragon handles merge to repos as well as commits to repos

Additional context: