git submodule summary: add --files option

git submodule summary: add --files option

git submodule summary is providing similar functionality for submodules as
git diff-index does for a git project (including the meaning of --cached).
But the analogon to git diff-files is missing, so add a --files option to
summarize the differences between the index of the super project and the
last commit checked out in the working tree of the submodule.

Signed-off-by: Jens Lehmann <redacted>
Signed-off-by: Junio C Hamano <redacted>