This check was accidentally disabled by a bad find command.
Fixes bug 32402; bugfix on 0.4.2.1-alpha.
Obviously correct changes to already reviewed code.
* Move the shellcheck script from the Makefile to its own script file
* Reformat the shellcheck script so it's easier to read and modify
* Call the shellcheck script from the pre-commit hook
Fixes bug 30967; not in any released version of Tor.