Pre-commit script in backup, cbbs and tools-common repositories exits quietly if git couchbase.tools.format.behaviour not set

If the git parameter couchbase.tools.format.behaviour is not set, the pre-commit script in the backup, cbbs and tools-common repositories (https://github.com/couchbase/backup/blob/master/.githooks/pre-commit)
will exit without an error message, and the commit won't take place.

The readme states that no config value should be treated the same way as if the config value was "commit"