check if the deploy directory is empty or not

[txchen]

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.

[txchen]

Thanks for making this great script. It fits very well for non-jekyll github pages scenarios!

When I try to play with the script, I found that if the target dir does not exist or if it is empty, we might get some weird results. I think it is better to check if the folder has some files in the beginning, details in the commit message, thanks a lot!

[X1011]

glad my script is helpful!

1. i'm not sure i follow how it may wipe out the gh-pages branch. could you give me an example?
2. yes, that's not a friendly error. thanks for the PR. i'll push it to master after a couple tweaks.
3. yes, that probably doesn't work. we should have tests for these edge cases, so i've created issue add tests #6: add tests, one of which should test "non-existent remote deploy_branch". i don't know if or when i'll get around to this, as i'm not even currently using this script.

[X1011]

oh, i think i know what you mean for the 1st one. it will wipe out the contents of deploy_branch, not the deploy_branch itself. i'll check that as a separate condition and let the user override with a flag.