Git diff says subproject is dirty

Update Jan. 2021, ten years later: "git diff"(man) showed a submodule working tree with untracked cruft as Submodule commit <objectname>-dirty, but a natural expectation is that the "-dirty" indicator would align with "git describe --dirty"(man), which does not consider having untracked files in the working tree as source of dirtiness. The inconsistency has been fixed with Git 2.31 (Q1 2021). See