问题描述：用“pod update”更新，遇到如下错误
Pull is not possible because you have unmerged files.
Please, fix them up in the work tree, and then use 'git add/rm <file>'
as appropriate to mark resolution, or use 'git commit -a'.问题解决：
A bug was found in libgit2 and they had to execute a force push on the specs repo. This is what broke everyone's CocoaPods setup.
You can find the official post about this issue on the CocoaPods blog :http://blog.cocoapods.org/Repairing-Our-Broken-Specs-Repository/
The recommended way to fix your setup is to execute the following commands :
$ pod repo remove master
$ pod setup
If that doesn't work, you can also delete manually all your cached specs :
$ rm -rf ~/.cocoapods/
$ pod setup