run different command error, because Pod::Config is a singleton.

[yishuiliunian]

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What did you do?

I use cocoapods as a gem lib. and define two Pod::Command subclass. first package an pospec, anthor publish a podspec.

when i run the two command, the second one is always break magical. Then i checkout the source of cocoapods. And i found the first one share the Pod::Config with the second one. But it is not i wanted.

The first pollute the second enviroment.

What did you expect to happen?

two command run correct. and do not use singleton with Pod::Config

What happened instead?

the second one crash!

CocoaPods Environment

### Stack

```
   CocoaPods : 1.5.3
        Ruby : ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-darwin18]
    RubyGems : 2.7.6
        Host : Mac OS X 10.14 (18A391)
       Xcode : 10.0 (10A255)
         Git : git version 2.17.1 (Apple Git-112)

[segiddins]

You can use Pod::Config.instance.with_changes({}) to scope changes to the config