[bug 1074315] Ignore deleted files for linting in pre-commit hook

Use mythmon's fix for this issue in kitsune repository from
https://github.com/mozilla/kitsune/pull/2252
Merge pull request #408 from lgp171188/1074315

Fix bug "1074315 -  fix bin/flake8_lint.sh script to not try to lint files that don't exist"
Get list of .py files to lint using just git diff
Merge pull request #409 from lgp171188/improve-getting-list-of-py-files-for-lint

Get list of .py files to lint using just git diff