Now that TFS 2015 comes with the same new Rest API of VS Team Services, I've taken a look at the API doc:
https://www.visualstudio.com/en-us/integrate/api/git/overview
One question naturally raised is that most queries do not expose an parameter for git branch or tag (e.g. download /path/to/my/file with tag 'release_v1.0'), which looks like a show stopper. As in my case, I need programatically pull out some source file under a certain branch/tag.
Is it not supported yet?