better mechanism for determining release revision

app/views/static/_footer.html.erb

@@ -1,6 +1,8 @@
 <footer id="page-footer">
   Running Danbooru <%= link_to "v#{Danbooru.config.version}", "https://github.com/r888888888/danbooru/releases" %>
-  (<%= Rails.application.config.x.git_hash %>)
+  <% if Rails.application.config.x.git_hash %>
+    (<%= Rails.application.config.x.git_hash %>)
+  <% end %>
   &ndash; <%= link_to "Rules", terms_of_service_path %>
   &ndash; <%= link_to "Contact", contact_path %>
   <% if CurrentUser.user.enable_post_navigation %>

config/application.rb

@@ -22,7 +22,11 @@ module Danbooru
     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
+    if File.exists?("#{config.root}/REVISION")
+      config.x.git_hash = File.read("#{config.root}/REVISION").strip
+    else
+      config.x.git_hash = nil
+    end
   end
 
   I18n.enforce_available_locales = false