I wrote this a little over a year ago, only (for the lulz) in Node rather than Go:

https://github.com/stuartpb/lhc

Mine allows using a custom word list in the commit message for the nonce.

Judging by this commit, I'm guessing gitbrute uses miniscule variations in the commit time instead.

EDIT: yep: https://github.com/bradfitz/gitbrute/blob/master/gitbrute.go