title: Problem with Mutlibranch Pipeline configured by jenkins DSL - repository name adding one unnecessary slash at the beggining
body: <p>I wanted to configure Multibranch Pipeline Job at Jenkins using <code>jenkins.yml</code> file.</p>
  
  <p>Here is fragment of my configuration:</p>
  
  <pre><code>multibranchPipelineJob('&lt;jenkins-job-name&gt;') {
          branchSources {
            github {
              (...)
              repository('&lt;user-group/repository&gt;')
            }
          }
  (...)
  }
  </code></pre>
  
  <p>When I deploy jenkins with configurated job like that, I have github repository URL address filled like that:
  <code>https://github.com//user-group/repository</code></p>
  
  <p>After github.com I have double slash and because of that automatic repository scanning is not working, I have something like that in logs:</p>
  
  <pre><code>[Fri May 22 13:39:00 UTC 2020] Starting branch indexing...
  13:39:00 Connecting to https://api.github.com using &lt;github-username&gt;/******
  ERROR: [Fri May 22 13:39:00 UTC 2020] Could not update folder level actions from source &lt;id&gt;
  hudson.AbortException: Invalid scan credentials when using &lt;github-username&gt;/****** to connect to /&lt;user-group/repository&gt; on https://api.github.com
      at org.jenkinsci.plugins.github_branch_source.GitHubSCMSource.retrieveActions(GitHubSCMSource.java:1884)
      at jenkins.scm.api.SCMSource.fetchActions(SCMSource.java:848)
      at jenkins.branch.MultiBranchProject.computeChildren(MultiBranchProject.java:600)
      at com.cloudbees.hudson.plugins.folder.computed.ComputedFolder.updateChildren(ComputedFolder.java:277)
      at com.cloudbees.hudson.plugins.folder.computed.FolderComputation.run(FolderComputation.java:164)
      at jenkins.branch.MultiBranchProject$BranchIndexing.run(MultiBranchProject.java:1034)
      at hudson.model.ResourceController.execute(ResourceController.java:97)
      at hudson.model.Executor.run(Executor.java:428)
  [Fri May 22 13:39:00 UTC 2020] Finished branch indexing. Indexing took 0.14 sec
  FATAL: Invalid scan credentials when using &lt;github-username&gt;/****** to connect to /&lt;user-group/repository&gt; on https://api.github.com
  Finished: FAILURE
  </code></pre>
  
  <p>Is this bug on Multibrach Pipeline Jobs handling?
  Is there any way to workaround it?</p>