combined diff seems to ignore diff attribute

I want to squelch the output of a particular file type because it's
too noisy (Mac OS X xib files). So I added:

*.xib -diff

to .git/info/attributes

which works fine except for --cc output. Is this a bug, oversight, or
my stupidity?

$ git log -p --cc
commit fdffb6081c2c09f55e2fdb6a02905054484965ed
Merge: 947da38 dcce084
Author: Jay Soffian [off-list ref]
Date:   Tue Sep 7 20:28:44 2010 -0400

    Merge branch 'side'

    * side:
      side

    Conflicts:
        file.xib

diff --cc file.xib
index 1f7391f,2299c37..0f8f0c9

--- a/file.xib
+++ b/file.xib

@@@ -1,1 -1,1 +1,1 @@@
- master
 -side
++master side

commit dcce0846b493edc0ab4301fae6c3910a6dd31ed8
Author: Jay Soffian [off-list ref]
Date:   Tue Sep 7 20:28:17 2010 -0400

    side

diff --git a/file.xib b/file.xib
index e69de29..2299c37 100644
Binary files a/file.xib and b/file.xib differ

commit 947da38b8ec84527aa60c0e8bca7279e6b2bb0af
Author: Jay Soffian [off-list ref]
Date:   Tue Sep 7 20:28:09 2010 -0400

    master

diff --git a/file.xib b/file.xib
index e69de29..1f7391f 100644
Binary files a/file.xib and b/file.xib differ

commit 419c5c0dc0ae4d548227d85ded2d8abc42001720
Author: Jay Soffian [off-list ref]
Date:   Tue Sep 7 20:27:49 2010 -0400

    initial

diff --git a/file.xib b/file.xib
new file mode 100644
index 0000000..e69de29