Weirdness with git change detection

I have a local git repo that's in some weird state where changes appear to be detected by "git diff" and prevent operations like "git checkout" from switching branches, but those changes are not removed by a "git reset --hard" or "git stash".

Here's an example of the behaviour, with "git reset --hard" failing to clear a diff in the index:

https://paste.debian.net/975811/

Happy to collect additional debugging information if it's useful.