Use defined colour for a language when available

This commit changes the colours of languages in the language graph to
the ones Linguist has defined.  When there is no colour defined for a
language in Linguist, it will fall back to the old method of finding
a colour.

app/controllers/projects/graphs_controller.rb

@@ -35,15 +35,19 @@ class Projects::GraphsController < Projects::ApplicationController
   def languages
     @languages = Linguist::Repository.new(@repository.rugged, @repository.rugged.head.target_id).languages
     total = @languages.map(&:last).sum
+    colors = Linguist::Language.colors.
+             select { |lang| @languages.include? lang.name }.
+             map { |lang| [lang.name, lang.color] }.
+             to_h
 
     @languages = @languages.map do |language|
       name, share = language
-      color = Digest::SHA256.hexdigest(name)[0...6]
+      color = colors[name] || "##{Digest::SHA256.hexdigest(name)[0...6]}"
       {
         value: (share.to_f * 100 / total).round(2),
         label: name,
-        color: "##{color}",
-        highlight: "##{color}"
+        color: color,
+        highlight: color
       }
     end