Apply reusable git builds patch

[meatballhat]

originally opened as travis-ci/gimme#151

[meatballhat]

/cc @philpennock

[philpennock]

LMK if you want me to take a look at rebasing etc, or if this is being handled elsewhere.

[meatballhat]

@philpennock Are you up for taking this? I'd love to see your work finally integrated ❤️

[philpennock]

It won't be "soon", but I can add it to my todo list.