Intermittent job failure due to "Unable to fetch the repository"

[samdoran]

Description of your issue:

Since Friday, June 14, 2019 we are occasionally but regularly seeing jobs fail because they are unable to fetch the repository from GitHub. This is a public repository. Restarting the job after waiting a few minutes usually resolves the issue.

A recent example: https://app.shippable.com/github/ansible/ansible/runs/128662/1/console

  ssh-agent bash -c 'ssh-add /tmp/ssh/01_deploy; git clone https://github.com/ansible/ansible.git /root/src/github.com/ansible/ansible' 20s  
  git config --get user.name || git config user.name 'Shippable Build' 0s  
  git config --get user.email || git config user.email 'build@shippable.com' 0s  
  rm -rf /root/src/github.com/ansible/ansible/shippable 0s  
  mkdir -p /root/src/github.com/ansible/ansible/shippable/testresults 0s  
  mkdir -p /root/src/github.com/ansible/ansible/shippable/codecoverage 0s  
  mkdir -p /root/src/github.com/ansible/ansible/shippable/buildoutput 0s  
  ssh-agent bash -c 'ssh-add /tmp/ssh/01_deploy; git fetch origin pull/58134/head' 1s  
Identity added: /tmp/ssh/01_deploy (/tmp/ssh/01_deploy)
fatal: Couldn't find remote ref pull/58134/head

retrying 1 of 3 times...
  ssh-agent bash -c 'ssh-add /tmp/ssh/01_deploy; git fetch origin pull/58134/head' 1s  
Identity added: /tmp/ssh/01_deploy (/tmp/ssh/01_deploy)
fatal: Couldn't find remote ref pull/58134/head

retrying 2 of 3 times...
  ssh-agent bash -c 'ssh-add /tmp/ssh/01_deploy; git fetch origin pull/58134/head' 0s  
Identity added: /tmp/ssh/01_deploy (/tmp/ssh/01_deploy)
fatal: Couldn't find remote ref pull/58134/head

retrying 3 of 3 times...
Unable to fetch the repository. If this is a private repository, please make sure that the repository still contains Shippable's deploy key. If the deploy key is not present in the repository, you can use the "Reset Project" button on the project settings page to restore it.