Scroll total scroll width instead of fixed value

Fixed value could potentially be not enough

Fix
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12198#note_37145886

app/assets/javascripts/repo/helpers/repo_helper.js

@@ -231,7 +231,7 @@ const RepoHelper = {
     setTimeout(() => {
       const tabs = document.getElementById('tabs');
       if (!tabs) return;
-      tabs.scrollLeft = 12000;
+      tabs.scrollLeft = tabs.scrollWidth;
     }, 200);
   },