Fetch repositories with --quiet option by default

What does this MR do?

This change will allow us to prevent deadlocks when dealing with extremely long outputs from git fetch. All the pipes (stdin, stdout, and stderr) have limited buffers, and when one fills up, everything is going to stop.

Are there points in the code the reviewer needs to double check?

I think this is the most boring solution, but we can try other approaches to read stdout and stderr simultaneously using thread or IO.select.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/28183

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10273

@smcgivern @rspeicher @brodock

lib/gitlab_projects.rb

@@ -207,6 +207,7 @@ class GitlabProjects
     cmd = %W(git --git-dir=#{full_path} fetch #{@name})
     cmd << '--prune'
     cmd << '--force' if forced
+    cmd << '--quiet'
     cmd << tags_option
     pid = Process.spawn(*cmd)