format-patch on permission change gives empty patch

When I ask git to format-patch a commit that is just a file
permission change, it ends up generating an empty file, not
even the commit message is included.

davem@sunset:~/src/GIT/net-2.6$ git show
commit a5dbc62ed61bf4cc57e22b78e5794880f1c74b90
Author: Joe Perches [off-list ref]
Date:   Wed Oct 6 17:32:49 2010 -0700

    Documentation/networking/ixgbevf.txt: Change file permissions to 644
    
    Signed-off-by: Joe Perches [off-list ref]
    Signed-off-by: David S. Miller [off-list ref]
You have new mail in /var/mail/davem
davem@sunset:~/src/GIT/net-2.6$ git format-patch HEAD^
0001-Documentation-networking-ixgbevf.txt-Change-file-per.patch
davem@sunset:~/src/GIT/net-2.6$ ls -l 0001-Documentation-networking-ixgbevf.txt-Change-file-per.patch 
-rw-r--r-- 1 davem davem 0 Oct  6 17:36 0001-Documentation-networking-ixgbevf.txt-Change-file-per.patch
davem@sunset:~/src/GIT/net-2.6$ git version
git version 1.7.3.1