Restrict hook to the changed files

[stevegrunwell]

Good recommendation from @bswatson:

When the Git hook runs it should only be run on files that have changed. This will reduce friction when implementing into an existing codebase as a file won't get checked (or at least not flagged) until something inside it has changed. Developers can still run ./vendor/bin/phpcs to scan the entire codebase, but this fix will make WP Enforcer much less of a "get everything in order before this is useful" tool.