git subtree pull says that the working tree has modifications, but git status says it doesn’t. What gives?

I just had the same problem.
From GIT source, error is thrown when command git diff-index HEAD returns result, even if git status says working tree is clean.

To avoid this error, in my case, I re-checkout the current branch of working tree , and everything seems OK : git checkout <branch>

It’s a bit confused, but if someone can explain why …