Fix note polling when a window has been hidden

`refresh` was called, `refreshing` was set to true, but then because
`document.hidden` was true, `getContent` was never called, and
`refreshing` never got reset to `false`, which stopped polling entirely
until refresh.

CHANGELOG

@@ -8,6 +8,8 @@ v 8.9.0 (unreleased)
   - Allow enabling wiki page events from Webhook management UI
   - Bump rouge to 1.11.0
   - Fix issue with arrow keys not working in search autocomplete dropdown
+  - Fix an issue where note polling stopped working if a window was in the
+    background during a refresh.
   - Make EmailsOnPushWorker use Sidekiq mailers queue
   - Fix wiki page events' webhook to point to the wiki repository
   - Don't show tags for revert and cherry-pick operations

app/assets/javascripts/notes.js.coffee

@@ -115,12 +115,14 @@ class @Notes
     , @pollingInterval
   refresh: =>
-    return if @refreshing is true
-    @refreshing = true
     if not document.hidden and document.URL.indexOf(@noteable_url) is 0
       @getContent()
   getContent: ->
+    return if @refreshing
+
+    @refreshing = true
+
     $.ajax
       url: @notes_url
       data: "last_fetched_at=" + @last_fetched_at