I just tried to checkout my master branch and ran into:

error: Untracked working tree file 'app.xcodeproj/project.xcworkspace/xcuserdata/u.xcuserdatad/UserInterfaceState.xcuserstate' would be overwritten by merge.

So, I tried to delete this file from git (I'd already added an expression in .gitignore to catch it) using:

git rm --cached app.xcodeproj/project.xcworkspace/xcuserdata/u.xcuserdatad/UserInterfaceState.xcuserstate

and got:

fatal: pathspec 'app.xcodeproj/project.xcworkspace/xcuserdata/u.xcuserdatad/UserInterfaceState.xcuserstate' did not match any files

So, at a bit of a loss. From my understanding the working file isn't the issue here. However, for completeness, a working file does exist. E.g. 

ls -l app.xcodeproj/project.xcworkspace/xcuserdata/u.xcuserdatad/UserInterfaceState.xcuserstate
-rw-r--r--  1 u  u  56061 24 Sep 12:42 app.xcodeproj/project.xcworkspace/xcuserdata/u.xcuserdatad/UserInterfaceState.xcuserstate
git git-rm
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