include git hash in footer

app/views/static/_footer.html.erb

@@ -1,5 +1,6 @@
 <footer id="page-footer">
   Running Danbooru <%= link_to "v#{Danbooru.config.version}", "https://github.com/r888888888/danbooru/releases" %>
+  (<%= Rails.application.config.x.git_hash %>)
   &ndash; <%= link_to "Rules", terms_of_service_path %>
   &ndash; <%= link_to "Contact", contact_path %>
   <% if CurrentUser.user.enable_post_navigation %>

config/application.rb

@@ -21,6 +21,8 @@ module Danbooru
     config.action_mailer.perform_deliveries = true
     config.log_tags = [lambda {|req| "PID:#{Process.pid}"}]
     config.active_record.raise_in_transactional_callbacks = true
+
+    config.x.git_hash = `git rev-parse --short HEAD`.strip
   end
 
   I18n.enforce_available_locales = false