title: GraphQL string concatenation or interpolation
body: <p>I'm using GitHub API v 4 to learn GraphQL. Here is a broken query to fetch blobs (files) and their text content for a given branch:</p>
  
  <pre><code>query GetTree($branch: String = "master") {
    repository(name: "blog-content", owner: "lzrski") {
      branch: ref(qualifiedName: "refs/heads/${branch}") {
        name
        target {
          ... on Commit {
            tree {
              entries {
                name
                object {
                  ... on Blob {
                    isBinary
                    text
                  }
                }
              }
            }
          }
        }
      }
    }
  }
  </code></pre>
  
  <p>As you see on line 3 there is my attempt of guessing interpolation syntax, but it does not work - I leave it as an illustration of my intention.</p>
  
  <p>I could provide a fully qualified name for a revision, but that doesn't seem particularly elegant. Is there any GraphQL native way of manipulating strings?</p>