Git branch DSL

[phodal]

https://github.com/nicoespeon/gitgraph.js/blob/18f6488d12532cd26ed6a4e9ee1da2488c099be0/packages/gitgraph-core/README.md

        // Simulate git commands with Gitgraph API.
        const master = gitgraph.branch("master");
        master.commit("Initial commit");

        const develop = gitgraph.branch("develop");
        develop.commit("Add TypeScript");

        const aFeature = gitgraph.branch("a-feature");
        aFeature
          .commit("Make it work")
          .commit("Make it right")
          .commit("Make it fast");

        develop.merge(aFeature);
        develop.commit("Prepare v1");

        master.merge(develop).tag("v1.0.0");
      }}