[Mac]brew updateしてもbrew doctorで警告がでる

$ brew doctor

Please note that these warnings are just used to help the Homebrew maintainers

with debugging if you file an issue. If everything you use Homebrew for is

working fine: please don't worry and just ignore them. Thanks!

Warning: Your Homebrew is outdated.

You haven't updated for at least 24 hours. This is a long time in brewland!

To update Homebrew, run `brew update`.

brew update すればいいのね　→　何回やってもbrew doctorで同じこと言われます。

/usr/local/Homebrew/README.md に

## Update Bug　なるものが書いてありました。

$ cd "$(brew --repo)" && git fetch && git reset --hard origin/master && brew update

をやると無事、解決しました。

$ brew doctor

Your system is ready to brew.

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OS X El Capitan(10.11.6)

iMac (21.5-inch, Late 2012)

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