Make diff view name getter less error prone

app/assets/javascripts/merge_request_tabs.js.coffee

@@ -179,11 +179,9 @@ class @MergeRequestTabs
 
     $.ajax(options)
 
+  # Returns diff view type
   diffViewMode: ->
-    # Gets diff view mode based from its ID
-    # See diff_btn helper method
-    btnID = $('a[id$="-diff-btn"].active').attr('id')
-    btnID.substr(0, btnID.indexOf('-'))
+    $('.diffs a[id$="-diff-btn"].active').data('view-type')
 
   expandViewContainer: ->
     $('.container-fluid').removeClass('container-limited')

app/helpers/diff_helper.rb

@@ -137,7 +137,7 @@ module DiffHelper
     # Always use HTML to handle case where JSON diff rendered this button
     params_copy.delete(:format)
 
-    link_to url_for(params_copy), id: "#{name}-diff-btn", class: (selected ? 'btn active' : 'btn') do
+    link_to url_for(params_copy), id: "#{name}-diff-btn", class: (selected ? 'btn active' : 'btn'), data: {view_type: name} do
       title
     end
   end