commit_subject: support for pattern matching in allowed prefixes

Currently the check only supports a hard-coded list of allowed_prefixes. In order to support third-party imports (vendoring) snapshot commits, we need to be able to match a pattern like ^[A-Za-z0-9_-]+ 20[0-9][0-9]-[0-9][0-9]-[0-9][0-9] \([0-9a-f]*\).