Fix error when issuing git pull on remote-only branch

What does this MR do?

This MR fixes the error returned by git when issuing a git pull against a branch that does not exist in the local copy of gemnasium-db.

Discussion

The approach I chose adds yet another git command to verify the state of the checkout. This is overly complicated and I'm in the search of something simpler and welcome suggestions.

Note: I broke up the conditional git pull out into a separate updateRef func as Update was getting a little hard to follow. I'm not sure if this is the best way. Looking for input here too.

What are the relevant issue numbers?

gitlab-org/gitlab#294296 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
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  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer