b:undo_ftplugin
Instead of re-writing the same condition each time, I put the function in an autoload plugin instead (which is going to be loaded just about every time I start writing code, but that's neither here nor there) to stop repeating myself.
I hit the problem when loading a fugitive window, but this should be done for all filetype plug-ins before trying to append to the variable.
The inline diff view in fugitive contains spaces, they shouldn't be highlighted to avoid distracting me.