check if the deploy directory is empty or not

An empty deploy directory would generate unexpected results:
* It may wipe out gh-pages branch, usually we don't expect that.
* In a newly cloned repo, if the folder does not exist, script would
  get error like:
  fatal: This operation must be run in a work tree
  The error is from line "git --work-tree "$deploy_directory" reset
  --mixed --quiet"
* If remote branch does not exist, 'deploy.sh -s' would also get wrong
  result.

To improve it, check if the directory is not empty in the beginning.