Allow for overriding only specific Mustache templates/partials. (#1719)

* Allow for overriding only specific templates/partials. Resolves  #1450.

lib/gollum/app.rb

@@ -22,6 +22,7 @@ require 'gollum/views/has_page'
 require 'gollum/views/has_user_icons'
 require 'gollum/views/pagination'
 require 'gollum/views/rss.rb'
+require 'gollum/views/template_cascade'
 
 require File.expand_path '../helpers', __FILE__
 
@@ -110,7 +111,11 @@ module Precious
       @wiki_title = settings.wiki_options.fetch(:title, 'Gollum Wiki')
 
       forbid unless @allow_editing || request.request_method == 'GET'
-      Precious::App.set(:mustache, {:templates => settings.wiki_options[:template_dir]}) if settings.wiki_options[:template_dir]
+
+      if settings.wiki_options[:template_dir]
+        Precious::Views::Layout.extend Precious::Views::TemplateCascade
+        Precious::Views::Layout.template_priority_path = settings.wiki_options[:template_dir]
+      end
 
       @base_url = url('/', false).chomp('/').force_encoding('utf-8')
       @page_dir = settings.wiki_options[:page_file_dir].to_s

lib/gollum/views/template_cascade.rb

@@ -0,0 +1,36 @@
+module Precious
+  module Views
+    module TemplateCascade
+      def template_priority_path
+        @@template_priority_path
+      end
+
+      def template_priority_path=(path)
+        @@template_priority_path = File.expand_path(path)
+        @template = nil
+      end
+
+      def first_path_available(name)
+        priority = File.join(template_priority_path, "#{name}.#{template_extension}")
+        default = File.join(template_path, "#{name}.#{template_extension}")
+        File.exists?(priority) ? priority : default
+      end
+
+      # Method should track lib/mustache/settings.rb from Mustache project.
+      def template_file
+        @template_file || first_path_available(template_name)
+      end
+
+      # Method should track lib/mustache.rb from Mustache project.
+      def partial(name)
+        path = first_path_available(name)
+        begin
+          File.read(path)
+        rescue
+          raise if raise_on_context_miss?
+          ""
+        end
+      end
+    end
+  end
+end