Remove unnecessary escaping.

While I can't say I fully understand escaping rules for Windows, I
understand them better now than I did when I copied that code from
another plugin and hoped for the best.

As far as I can tell, this escaping code was written to be used when
manually constructing a system command to execute – which isn't how this
plugin is written.  So this code shouldn't be here.

autoload/diff.vim

@@ -23,7 +23,6 @@ function! diff#run_diff(realtime, use_external_grep)
   endif
 
   let cmd .= ')'
-  let cmd = utility#escape(cmd)
 
   if a:realtime
     let diff = system(utility#command_in_directory_of_file(cmd), utility#buffer_contents())