Add simple logging.

autoload/gitgutter/diff.vim

@@ -134,9 +134,11 @@ function! gitgutter#diff#run_diff(realtime, preserve_full_diff)
           \ 'on_stderr': function('gitgutter#handle_diff_job'),
           \ 'on_exit':   function('gitgutter#handle_diff_job')
           \ })
+    call gitgutter#debug#log('[job_id: '.job_id.'] '.cmd)
     if job_id < 1
       throw 'diff failed'
     endif
+
     call gitgutter#utility#pending_job(job_id)
     return 'async'
   else