I’m getting this strange output from ‘git diff’

bash$ git diff

diff --git a/libraries/exceptions b/libraries/exceptions

--- a/libraries/exceptions

+++ b/libraries/exceptions

@@ -1 +1 @@

-Subproject commit fe4166f8d23d8288ef2cbbf9e36118b6b99e0d7d

+Subproject commit fe4166f8d23d8288ef2cbbf9e36118b6b99e0d7d-dirty

No amount of git submodule update makes it go away.  Any ideas?

Thanks

Simon