Don't barf on readonly files if contents haven't changed

Fix for #94
I thought about it but I'm not sure we want to start taking encoding or line endings into account. For systems that are checking-out readonly, they shouldn't be messing with what was checked in.

I know that TFS in particular will not mess with it; I don't know for sure about Perforce or others. That said, if there's a request for it, then we can handle it then...