check-commit: try to curb bad commit summary keywords The goal of commit summary keywords is to help us sort, categorize, and filter our voluminous commits for our release notes in a way that's helpful and meaningful to end users. Lately, there have been a huge number of "keywords" that are neither words nor particularly key. This patch tries to discourage that by narrowing the allowed characters to alphanumeric. In particular, it doesn't allow "." (method, function names, and file extensions) and "/" (filenames). It also gives a short reminder of what a keyword ought to be.