Enables frozen string for the following: * lib/gitlab/fogbugz_import/**/*.rb * lib/gitlab/gfm/**/*.rb * lib/gitlab/git/**/*.rb * lib/gitlab/gitaly_client/**/*.rb * lib/gitlab/gitlab_import/**/*.rb * lib/gitlab/google_code_import/**/*.rb * lib/gitlab/gpg/**/*.rb * lib/gitlab/grape_logging/**/*.rb * lib/gitlab/graphql/**/*.rb * lib/gitlab/graphs/**/*.rb * lib/gitlab/hashed_storage/**/*.rb * lib/gitlab/health_checks/**/*.rb Partially address gitlab-org/gitlab-ce#47424.
Having this in a concern allows us to reuse it for different single purpose classes that call out to git without going through the repository every time.
