Moving around git2r reference

[rmbielby]

Pull request overview

ShinyApps doesn't seem to have access to git libraries, so running library(git2r) breaks the dashboard. I've moved the git2r reference to be completely contained just in the commit hooks script.

Pull request checklist

Please check if your PR fulfils the following:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Tests have been run locally and are passing (run_tests_locally())
• Code is styled according to tidyverse styling (checked locally with tidy_code())