Output of 'diff' and 'diff --raw' not the same for modified submodule

If a submodule has new commits, then 'git diff' and 'git diff --raw'
report different outputs.

For example, the following output is given after adding a new commit
to a submodule:

$ git version
git version 1.7.10.rc4

$ git diff

diff --git a/lib/tst-mergetool b/lib/tst-mergetool
index 9917ea0..5bc1908 160000
--- a/lib/tst-mergetool
+++ b/lib/tst-mergetool

@@ -1 +1 @@
-Subproject commit 9917ea0c228a0c24153da38df954f8a9d78cafac
+Subproject commit 5bc19083c436aaa85762cdc309e62ed98f5cc276

$ git diff --raw --no-abbrev
:160000 160000 9917ea0c228a0c24153da38df954f8a9d78cafac
0000000000000000000000000000000000000000 M  lib/tst-mergetool


'git diff' shows the new SHA1 for the submodule, but 'git diff --raw'
shows the NULL ID.  From the diff man page, I understand that the NULL
ID indicates "creation, unmerged or 'look at the work tree'", but I
expected the output of two commands to match.

Is this the intended behavior?

Thanks,
Tim