WIP: only send events with Changelog files to walkEvents

Most events aren't even Commits, so we can reject them quickly. This seems to leave a lot of the threads idle as we don't (or can't) start new goroutines fast enough. Instead, filter the list first and then run them.

This is functional, but causes DoChangelogs to take ~8 times longer to complete. Perhaps there's more lock contention or something? There's definitely a lot more memory use, and swapping was part of the slowdown.