Copy LFS objects when a repository is pull mirrored

What does this MR do?

Copies LFS objects into the repository when it is pull mirrored.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/10871

app/views/projects/mirrors/_instructions.html.haml

@@ -7,7 +7,7 @@
     %li
       - minutes = Gitlab.config.gitlab_shell.git_timeout / 60
       = _("The update action will time out after %{number_of_minutes} minutes. For big repositories, use a clone/push combination.") % { number_of_minutes: minutes }
-    %li= _('The Git LFS objects will <strong>not</strong> be synced.').html_safe
+    %li= _('The Git LFS objects will <strong>not</strong> be synced in push mirrors.').html_safe
     %li
       = _('This user will be the author of all events in the activity feed that are the result of an update,
       like new branches being created or new commits being pushed to existing branches.')