git diff --submodule: Show detailed dirty status of submodules

git diff --submodule: Show detailed dirty status of submodules

When encountering a dirty submodule while doing "git diff --submodule"
print an extra line for new untracked content and another for modified
but already tracked content. And if the HEAD of the submodule is equal
to the ref diffed against in the superproject, drop the output which
would just show the same SHA1s and no commit message headlines.

To achieve that, the dirty_submodule bitfield is expanded to two bits.
The output of "git status" inside the submodule is parsed to set the
according bits.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>