git works as a deployment tool, but not in all cases. It's useful to deploy code, but when writing Java or Go, you usually don't deploy the code, you deploy compiled binaries. Currently, tsuru forces you to deploy your code (you can create a git repository versioning the binaries, and do some local magic to compile your source, put it in the repository and push it - like creating a "deploy" target in your Makefile, but that's an ugly workaround).

In the future, tsuru will probably support other ways of deployment.

git absolutely is a deploy tool.

And when they force you to use git they always forget to also support mercurial...

Git can be a deployment tool by using the post-update hook in a server-side repo. In my setups it also runs tests in a "test" repo before deciding to push into a "production" one.