Two things that could be handy to know if you are trying to use TFS Git and Xamarin Studio on Mac.
1. The username cannot contain @
2. If you mistype your password, you have to remove the keychain entry (http://forums.xamarin.com/discussion/2514/change-version-control-credentials#latest)
Both will fail with an UnauthorizedException and no further info.
And for the first part to work you must specify an alternative login. This is well documented when you've created your repository however.
EDIT: Still some trouble with pushing to remote GIT. Everytime I want to push I have to:
1. Delete the keychain entry
2. Enter my credentials
3. An error message is displayed, but the changes are pushed anyhow
Hopefully this will be fixed in the 4.0.4 release!