The GitLab::Git::Repository class had two methods which aren't really
used in production. `#create_branch` and `#remove_branch` now leverage
WriteRef. The behaviour does change now, in the case of `#create_branch`
it doesn't throw an error anymore if the branch already exists. That's
actually not a problem as all call sites are import related, in which
case there's only one branch to be imported.

`#delete_branch` is only used in tests. The runtime errors aren't
depended on in that case. Again, this change in behaviour doesn't
influence GitLab-Rails.

When importing, we want to create `refs/heads/:source_branch` in the
repository for each MR. Since the imported project already contains
the SHA, there's no need to use `fetch_source_branch` - we can just
create the ref directly.

Suggests to use a JSON structured log instead

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/54102

Suggests to use a JSON structured log instead

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/54102

Enables frozen string for the following:

* lib/gitlab/hook_data/**/*.rb
* lib/gitlab/i18n/**/*.rb
* lib/gitlab/import/**/*.rb
* lib/gitlab/import_export/**/*.rb
* lib/gitlab/kubernetes/**/*.rb
* lib/gitlab/legacy_github_import/**/*.rb
* lib/gitlab/manifest_import/**/*.rb
* lib/gitlab/metrics/**/*.rb
* lib/gitlab/middleware/**/*.rb

Partially addresses gitlab-org/gitlab-ce#47424.

This adds various foreign keys and indexes to the "merge_requests" table
as outlined in https://gitlab.com/gitlab-org/gitlab-ce/issues/31825.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/31825

This adds various foreign keys and indexes to the "merge_requests" table
as outlined in https://gitlab.com/gitlab-org/gitlab-ce/issues/31825.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/31825

Added unit test and updated integration spec to test for this as well.