Piotr Krukowiecki [off-list ref] writes:
quoted
Sure. The index contains multiple competing versions, which you can
see with "git ls-files -u". To compare the working tree and one of
those use "git diff --base / --ours / --theirs" (or -1 / -2 / -3).
Thanks, didn't know about that (and it's not documented in diff man page).
These "compare a particular stage against working tree file" modes
only make sense when you are comparing the index and the working tree, so
the description is in "git diff-files" manpage but hasn't been ported to
the documentation of generic "git diff" frontend.
Perhaps we should.