Unfortunately using the Git integration in UE4 is highly experimental in 4.7.x. We do not have merge workflows set up for all asset types (only Blueprints at the moment, and that is early) so anyone using this plugin to work collaboratively will run into issues very quickly.
Historically UE4 has worked on an ‘exclusive checkout’ model, where assets cannot be edited by multiple people at once. We support this in the Perforce and SVN plugins, but Git does not have this concept.
Also unfortunately, we do not have any planned work to address collaborative workflow issues using Git in the near future.