Apple has introduced a new project-related type of file in Xcode 5: "xccheckout".
This file is located in the ".xcodeproj/project.xcworkspace/xcshareddata/" directory, and it seems that it is related to the project's version control system.
An example file is here: http://pastebin.com/5EP63iRa
I suppose that this type of file should be ignored under VCS, but I'm not sure.
So here are the questions:
You should check in an Xcode 5 .xccheckout file; in general, files in xcshareddata should be committed.
An .xccheckout file contains metadata about what repositories are used in a workspace. For a single project in a single repository that doesn't make much difference. But if you're using a workspace that has multiple projects from different repositories, the presence of an .xccheckout file in the workspace allows Xcode to know what all of the components that make up a workspace are and where to get them.
