研究两天终于在stackoverflow上找到答案了。

Failure while executing: git pull -q origin refs/heads/master:refs/remotes/origin/master

更新brew 报这个错误的时候，

You need to do the following:

cd $(brew --prefix)

rm Library/Formula/argp-standalone.rb

rm Library/Formula/cocot.rb

And then do the

git fetch origin

git reset --hard origin/master

brew update

－－－－－－－－－或者－－－－－－－－－

cd `brew --prefix`

$ git remote add origin https://github.com/Homebrew/homebrew.git

$ git fetch origin

$ git reset --hard origin/master

$ brew update

Already up-to-date.