Refactor shell script syntax and consistency

- Add a missing space for styling
- Replace legacy "\`...\`" with `$(...)`
- Quote variable to prevent word splitting
- Use builtin `command -v` instead of non-standard `which`
- Add two missing `>&2` redirection for error/warning message

script/sdist

@@ -3,13 +3,13 @@
 set -e
 
 if ! git diff-index --quiet HEAD --; then
-    echo "Changes in local directory, commit or clear"
+    echo "Changes in local directory, commit or clear" >&2
     exit 1
 fi
 
 SHA=$(git rev-parse HEAD)
 python setup.py sdist
-TARBALL=dist/octodns-$SHA.tar.gz
-mv dist/octodns-0.*.tar.gz $TARBALL
+TARBALL="dist/octodns-$SHA.tar.gz"
+mv dist/octodns-0.*.tar.gz "$TARBALL"
 
 echo "Created $TARBALL"