@DHowett ooh, apparently that works, thank you!
But how on earth did you find that? It's taken me ten minutes _even_ knowing it was there. Even after I knew about it I wasn't able to find it by looking in obvious parts of the git documentation.
I eventually grepped the git source¹, and found references to this kind of thing in release notes under the name "magic pathspec". In the actual git man pages that phrase occurs just once, in 'git-glossary', under the section on 'pathspec' in general – and aha, that _is_ where this kind of thing is documented.
I see that it's not in the man page for 'git diff' because it's more general than that, like the way that the man page for 'git rev-parse' is where you find all the general-purpose stuff about how to specify commits in lots of different ways (like ^ and ~3 etc).
But I'd never have thought to look up a description of _functionality_ in the glossary: that's surely where you go for "what does this word even mean?", and if I'd seen the word "pathspec", I would have assumed that I already knew what it meant. Plus, 'git diff' doesn't even use the word "pathspec" at all – it just says "path"!
@b0rk, if you're still working on improving git's own documentation, something just jumped to the top of my own list of things that could use a little improvement. :-)
¹ imagine I said all of this in Arthur Dent's sarcastic tones from the start of Hitchhikers