Just thought of one more way to do this which is neater - define functions on a simpler slice type:

https://play.golang.org/p/0YcxA-Ufs-

So your sorting becomes quite neat:

    sort.SliceStable(changes,changes.byLines)
    sort.SliceStable(changes,changes.byUser)

I think I might use it this way (haven't actually used this in a real app yet).