[Command] Remove deprecated pod push
- fixed configuration specific headers
Modify add_xcode_base_configuration to respect existing base
configurations:
 - Do not change the base configuration when it is already set
 - Notify the user when there is an existing base configuration

Change string quoting from double to single quotes.

Split lines longer than 80 characters into multiple lines.

Quote initial string fragment with single quotes rather than double quotes.
[Changelog] Add link to issue
[CONTRIBUTING] Clarify responsibility more.

We have only so much time in a day, it’s time to take stronger stances on support.
[Pod Installer Source] Ensure `.git` is actually removed

Clarify why .git/ is removed from comparison
[XCConfigIntegrator] Update descriptions to be more verbose

Thanks @alloy!
Merge pull request #2581 from CocoaPods/use_base_configuration

Modify add_xcode_base_configuration to respect existing base configurations
[Downloader] ensure that a downloader failure results in a DownloaderError so the Downloader can catch it

Helpful for https://github.com/CocoaPods/cocoapods-downloader/pull/36
[SourcesManager] Ensure the master repo is shallow cloned

Closes https://github.com/CocoaPods/CocoaPods/issues/2586.
[CONTRIBUTING] Add no-nos about littering.
Remove references to Source#data_provider
Merge pull request #2600 from CocoaPods/remove-data-provider

Remove references to Source#data_provider
[TargetIntegrator] Ensures that the user project is not saved when there are no user targets integrated.
Ensure that we raise when attempting to stub an undefined method
[IPC::List] Fix a crash caused by calling an old method
Merge pull request #2601 from CocoaPods/unnecessary-save

[TargetIntegrator] Ensures that the user project is not saved when there are no user targets integrated.
[Installer] Fix issue inheriting pods into empty target

Fixes https://github.com/CocoaPods/CocoaPods/issues/2591
[Resolver] Take into account versions from all sources (CocoaPods/CocoaPods#2556)
[Resolver] Added tests, notice if the required version exists in multiple sources (CocoaPods/CocoaPods#2556)