pre-commit hook: drop git stash in favor of git diff
pre-commit hook: drop git stash in favor of git diff

there are edge cases where git stash digests working directory and
refuses to give it back. it happened to me several times, unfortunately
i wasn't able to replicate this exact behaviour later. iirc @abuxton
also mentioned it happening, and courseware troubleshooting mentions
disappearing CWD.

for those reasons i propose dropping stash and re-implementing it using
diff. hook is modeled on this blog post:
http://jakemccrary.com/blog/2015/05/31/use-git-pre-commit-hooks-to-stop-unwanted-commits/

additionally it validates 1st commit to repo, which original hook
ignored.

dylanratcliffe/pltraining-classroom,carthik/pltraining-classroom,binford2k/pltraining-classroom,carthik/pltraining-classroom,dylanratcliffe/pltraining-classroom,binford2k/pltraining-classroom,joshsamuelson/pltraining-classroom,joshsamuelson/pltraining-classroom,dylanratcliffe/pltraining-classroom,puppetlabs/pltraining-classroom,samuelson/pltraining-classroom,puppetlabs/pltraining-classroom,samuelson/pltraining-classroom,fnaard/pltraining-classroom,samuelson/pltraining-classroom,carthik/pltraining-classroom,fnaard/pltraining-classroom,spidersddd/pltraining-classroom,binford2k/pltraining-classroom,fnaard/pltraining-classroom,puppetlabs/pltraining-classroom,joshsamuelson/pltraining-classroom,spidersddd/pltraining-classroom,spidersddd/pltraining-classroom