A few general categories of fixes:
* Sprinkle lots of quotes everywhere.
* Don't use environment variables in the format string to printf, instead pass them as arguments.
* Don't use backticks to execute commands (it's deprecated), use the new "$(...)" syntax instead.
Implement support for skipping API comparison by applying a label to a pull
request.
This also required some refactoring to move existing code to fetch the labels
for a pull request to a separate script.