Link current commit if running inside a git repo.

Useful when running `rails server` straight from a dev repo.

config/application.rb

@@ -24,6 +24,8 @@ module Danbooru
 
     if File.exists?("#{config.root}/REVISION")
       config.x.git_hash = File.read("#{config.root}/REVISION").strip
+    elsif system("type git && git rev-parse --show-toplevel")
+      config.x.git_hash = %x(git rev-parse --short HEAD).strip
     else
       config.x.git_hash = nil
     end