You might want to consider using something like Bonobo Git Server that is designed to work with AD authentication:
- setup it with your IIS user management
- Windows and Basic authentication modes support
- use Active Directory with git to manage your access
Another option is Gitblit, which supports LDAP authentication:
Gitblit supports additional authentication mechanisms aside from it's internal one.
- LDAP authentication
- Windows authentication
- PAM authentication
- Htpasswd authentication
- Redmine auhentication
- Salesforce.com authentication
- Servlet container authentication
Note that Gitblit's support for "Windows authentication" isn't the same as AD authentication:
Windows authentication is based on the use of Waffle and JNA. It is known to work properly for authenticating against the local Windows machine, but it is unclear if it works properly with a domain controller and Active Directory.