pod install或者pod setup 报错如下：
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[!] An error occurred while performing `git pull` on repo `master`.
[!] /usr/bin/git pull --ff-only
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fatal: Not possible to fast-forward, aborting.
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进入~/.cocoapods/repos/master 删除 master
重新pod setup
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详情请见:https://github.com/CocoaPods/CocoaPods/issues/776
https://github.com/CocoaPods/CocoaPods/issues/948
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解决问题。