Cannot see the unstaged changes git filter-branch is complaining about

Question

On a new repo clone, git filter-branch complains that I have unstaged changes, eg

$ git filter-branch --subdirectory-filter plugins/hypervisors/hyperv -- --all 
Cannot rewrite branches: You have unstaged changes.

However, these git status says that there are no changes:

$ git status
# On branch master
nothing to commit (working directory clean)

Where do I look to find out what the changes are?

Answer 1

Resolved by wiping the stash, eg

git stash drop

Source

Would be interesting to know where the changes to the stash came from.