title: Jenkinsfile from SCM and @script/@tmp projects
body: <p>I have configured a jenkins multibranch pipeline project and I want to use Jenkinsfile from SCM. The suggested way is to just put a Jenkinsfile into my project. Jenkins will then bootstrap a job by cloning the repo and run the Jenkinsfile. I noted however, that the workspace then contains my project cloned three times: workspace/myproject and workspace/myproject@script, and workspace/myproject@tmp. I'd think that one clone would be sufficient? When I look at the build output, it sais something along:</p>
  
  <pre><code>Started by user felix
  Checking out git file:///C:/users/felix/git/myproject.git into C:\Program Files (x86)\Jenkins\workspace\myproject_local-win@script to read Jenkinsfile
  // then all the git stuff..
  Running on Jenkins in C:\Program Files (x86)\Jenkins\workspace\myproject_local-win
  // then the pipeline
  </code></pre>
  
  <p>So it clones into @Script to get the Jenkinsfile, and then runs it in a different clone? I can't see this explained anywhere, which makes me feel uncomfortable. So, what's the rationale?</p>