https://gitlab.com/gitlab-org/gitlab-foss/merge_requests/31741, shipped
in GitLab 12.2, reduced Gitaly RPCs by generating commit data only if
hooks or services were actually active. However, GitLab plugins
(https://docs.gitlab.com/ee/administration/plugins.html), are executable
programs that get launched inside `SystemHooksService#execute_hooks`
regardless of whether any hooks are actually active. As a result of this
change, users have to configure a Webhook for plugins to fire.

We restore the previous behavior by checking whether plugins are
available and executing `SystemHooksService#execute_hooks` if any
plugins are available.

Closes https://gitlab.com/gitlab-org/gitlab/issues/194131