The GIT integration in Visual Studio 2017 seems to have regressed back to the initial buggy state in Visual Studio 2015.
A new issue I had when starting VS2017 as an administrator was:
Git failed with a fatal error fatal: unable to access '\/.config/git/config': Invalid argument
This was visible in the output window, if you miss it you’re clueless.
The result is the GIT integration did not work at all and I had to fall back to VS2015 when I needed a UI to to GIT operations (e.g. merging)
You can fix this by setting an environment variable HOME that points to your folder with git repositories.