More efficient build farm animal wakeup?

[Thomas Munro]

Hi,

Is there a way to find out about new git commits that is more
efficient and timely than running N git fetches or whatever every
minute in a cron job?  Maybe some kind of long polling where you send
an HTTP request that says "I think the tips of branches x, y, z are at
111, 222, 333" and the server responds when that ceases to be true?