title: Checkout git submodule from azure pipeline
body: <p>I am currently working on an azure pipeline.
  Within my main github repo (repo A), I have another github repo added as a sub module. (repo B)</p>
  <p>My goal is to checkout the sub-module at the start of the pipeline with the following YAML:</p>
  <pre><code>stages:
  - stage: checkout
    jobs:
    - job: checkout
      steps:
        - checkout: self
          submodules: true
          persistCredentials: true
  </code></pre>
  <p>This then attempts to checkout the sub-module, but ends with the following error:</p>
  <pre><code>Cloning into '/home/vsts/work/1/s/devops-scripting'...
  fatal: could not read Username for 'https://github.com': terminal prompts disabled
  fatal: clone of 'https://github.com/sourcerepo/devops-scripting.git' into submodule path '/home/vsts/work/1/s/devops-scripting' failed
  </code></pre>
  <p>It seems to be an issue with using an incorrect user/password - if i was pushing i could simply use supply user/pass parameters, however this doesn't seem to work for checking out.</p>
  <p>How can i update a submodule via an azure pipeline?</p>