I have found articles regarding how to install Git on a Windows server and use SSH (such as CopSSH) for authentication. I am a little surprised by this as I remember readin
You could use Redmine to control user access to Git repositories. In turn, Redmine natively supports authentication with Active Directory. Thus, you have Active Directory authentication for your Git server. We use this setup without problems.
Just a remark in case you don't want to use Redmine. Redmine is in my opinion the best open source project management application out there, and it's really worth using even if you don't need the AD authentication.