Teach diff that modified submodule directory is dirty

Teach diff that modified submodule directory is dirty

A diff run in superproject only compares the name of the commit object
bound at the submodule paths.  When we compare with a work tree and the
checked out submodule directory is dirty (e.g. has either staged or
unstaged changes, or has new files the user forgot to add to the index),
show the work tree side as "dirty".

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