Update UpdateAllMirrorsWorker to respect update frequencies

It now runs every 15 minutes by default.

app/workers/update_all_mirrors_worker.rb

@@ -4,7 +4,7 @@ class UpdateAllMirrorsWorker
   def perform
     fail_stuck_mirrors!
-    Project.mirror.each(&:update_mirror)
+    Project.mirror.each { |project| project.update_mirror(force: false) }
   end
   def fail_stuck_mirrors!

config/gitlab.yml.example

@@ -186,9 +186,10 @@ production: &base
     historical_data_worker:
       cron: "0 12 * * *"
-    # Update mirrored repositories
+    # Update repository mirrors, if necessary, every 15 minutes. Mirrors with an
+    # update frequency higher than 15 minutes will only be updated as-needed.
     update_all_mirrors_worker:
-      cron: "0 * * * *"
+      cron: "*/15 * * * *"
     # In addition to refreshing users when they log in,
     # periodically refresh LDAP users membership.

spec/workers/update_all_mirrors_worker_spec.rb

@@ -14,7 +14,8 @@
       create(:empty_project, :mirror)
       create(:empty_project)
-      expect_any_instance_of(Project).to receive(:update_mirror).once
+      expect_any_instance_of(Project).
+        to receive(:update_mirror).with(force: false).once
       described_class.new.perform
     end