Before trying the official release of Visual Studio 2017, I installed Visual Studio 2017 RC and found that my existing Git Repository is not recognized. Even though I made sure that the Source Control plugin is set to Git as shown in the following picture.
Then, I thought, maybe it is just because of RC edition. Anyway, today official version of Visual Studio 2017 released and I have installed again. I see the same problem. Did anyone face the same issue? What is the solution to this problem? I do not think I need to install Git Extension separately as looks like Visual studio already has that extension preinstalled.
I had the same issue when running VS 2017 in administrator mode and attempting to open a repository in VSTS git. This post actually helped me find the answer
What worked for me was to create an environment variable "HOME" with the value of "C:\" (my source code directory). Setting this environment variable and restarting VS 2017 seems to have fixed it. All the best!