Since the file is modified by both, Either you need to add it by
git add Whereami.xcodeproj/project.xcworkspace/xcuserdatauser.xcuserdatad/UserInterfaceState.xcuserstate
Or if you would like to ignore yoyr changes, then do
git reset HEAD Whereami.xcodeproj/project.xcworkspace/xcuserdatauser.xcuserdatad/UserInterfaceState.xcuserstate
After that just switch your branch.This should do the trick.