Convert switch icon into icon font

CHANGELOG

@@ -3,6 +3,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.11.0 (unreleased)
   - Fix the title of the toggle dropdown button. !5515 (herminiotorres)
   - Improve diff performance by eliminating redundant checks for text blobs
+  - Convert switch icon into icon font (ClemMakesApps)
   - Remove magic comments (`# encoding: UTF-8`) from Ruby files. !5456 (winniehell)
   - Fix CI status icon link underline (ClemMakesApps)
   - Cache the commit author in RequestStore to avoid extra lookups in PostReceive

app/assets/stylesheets/pages/commits.scss

 .commits-compare-switch {
   @include btn-default;
   @include btn-white;
-  background: image-url("switch_icon.png") no-repeat center center;
-  text-indent: -9999px;
   float: left;
   margin-right: 9px;
 }

app/views/projects/compare/_form.html.haml

 = form_tag namespace_project_compare_index_path(@project.namespace, @project), method: :post, class: 'form-inline js-requires-input' do
   .clearfix
     - if params[:to] && params[:from]
-      = link_to 'switch', {from: params[:to], to: params[:from]}, {class: 'commits-compare-switch has-tooltip', title: 'Switch base of comparison'}
+      = link_to icon('exchange'), {from: params[:to], to: params[:from]}, {class: 'commits-compare-switch has-tooltip', title: 'Switch base of comparison'}
     .form-group.dropdown.compare-form-group.js-compare-from-dropdown
       .input-group.inline-input-group
         %span.input-group-addon from