[fix] check-format.sh script to also work with git

the check-format.sh script was originally written for a SVN-based workflow, only checking the formatting of changed files by default. This functionality was now added for git-based workflows, removing the need to call "-a" to examine all files.

Also the error codes were a bit touched, so that the number of files that need formatting are returned. This is theoriteically limited by shell error codes that must be within the range [0-255], but should practically not be an issue.