Reorder MergeRequestTabs constructor

The tab was being activated before we set the `[commits|diffs]Loaded`
variable, so even when the `/diffs` route is accessed directly, like
from the "Side-by-side" link, the tab was being loaded from its default
source.

Fixes #1887

app/assets/javascripts/merge_request_tabs.js.coffee

@@ -49,13 +49,13 @@ class @MergeRequestTabs
     # Store the `location` object, allowing for easier stubbing in tests
     @_location = location
 
-    @bindEvents()
-    @activateTab(@opts.action)
-
     switch @opts.action
       when 'commits' then @commitsLoaded = true
       when 'diffs'   then @diffsLoaded = true
 
+    @bindEvents()
+    @activateTab(@opts.action)
+
   bindEvents: ->
     $(document).on 'shown.bs.tab', '.merge-request-tabs a[data-toggle="tab"]', @tabShown