macOS 10.12 (Sierra) にしてから brew upgrade が実行されない

症状

$ brew upgrade
Warning: You are using OS X 10.12.
We do not provide support for this pre-release version.
You may encounter build failures or other breakages.
Please create pull-requests instead of filing issues.

$ brew -v
Homebrew 0.9.9 (git revision 06fe; last commit 2016-08-10)
Homebrew/homebrew-core (git revision b163b; last commit 2016-08-10)

原因

Homebrewの"Update Bug"

解決方法

cd $(brew --repo); git fetch; git reset --hard origin/master

出典