Fetch repositories with --quiet option by default

This 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.

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)