Can't understand the behaviour of git-diff --submodule

From: Francis Moreau <francis.moro@gmail.com>
Date: Oct 26, 2012, 14:07 UTC
Message-ID: <CAC9WiBgzbsury2f9FyAu=Pgn31f2uCtq7AvsVWGWEwoV6KbyjA@mail.gmail.com>

Hi,

I'm trying to use the --submodule switch with git-diff but doesnt understand the following behaviour:

$ git diff 2c9a257718d1803de720f95766ff256d33accad5 HEAD

diff --git a/configs b/configs
index 16c6a89..ce12289 160000
--- a/configs
+++ b/configs
@@ -1 +1 @@
-Subproject commit 16c6a89f245f0eed7fb0bce8e027c59fcf1d543e
+Subproject commit ce12289c5bfca7b2c423d9f1871c13ad1ba1dc32

but adding the --submodule option gives:

$ git diff --submodule=log 2c9a257718d1803de720f95766ff256d33accad5 HEAD
Submodule configs 16c6a89...ce12289 (commits not present)

Could anybody enlight me ?

-- 
Francis