Merge branch 'jl/diff-submodule-ignore'

* jl/diff-submodule-ignore:
  Teach diff --submodule that modified submodule directory is dirty
  git diff: Don't test submodule dirtiness with --ignore-submodules
  Make ce_uptodate() trustworthy again

diff --cc diff-lib.c
Simple merge

diff --cc diff.c
Simple merge