diff: do not show submodule with untracked files as "-dirty"

diff: do not show submodule with untracked files as "-dirty"

Git diff reports a submodule directory as -dirty even when there are
only untracked files in the submodule directory. This is inconsistent
with what `git describe --dirty` says when run in the submodule
directory in that state.

Make `--ignore-submodules=untracked` the default for `git diff` when
there is no configuration variable or command line option, so that the
command would not give '-dirty' suffix to a submodule whose working
tree has untracked files, to make it consistent with `git
describe --dirty` that is run in the submodule working tree.

And also make `--ignore-submodules=none` the default for `git status`
so that the user doesn't end up deleting a submodule that has
uncommitted (untracked) files.

Signed-off-by: Sangeeta Jain <sangunb09@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>