I feel strongly that it should be able to be set be repo-specific (and globally). Having it only per branch defeats a big part of it. Should be straightforward git config to have a repo-wide rule but still allow disabling it for e.g. 'gh-pages', I hope?
I don't have that much of an opinion on per-repo / per-branch
configuration but I just wanted to point out that some branches would
benefit from the ability to turn this off. Perhaps it could be a two
layered setting that could be both configured per-repo / per-branch
but I don't know if this makes it too complicated.