bug with submodule and assume-unchanged

Hello,

Marking a submodule with update-index --assume-unchanged breaks diff.

Consider the script in http://pastebin.ca/2082543. The last 'diff'
output follows. Note that the diff for 'afile' is displayed, then sub
breaks the operation, and the diff for 'test' (or any other file that
comes after 'sub') is not displayed at all.

$:/tmp/git$ cd super/
$:/tmp/git/super$ git diff
fatal: unable to read c17499ae216dfba9538ed7f94ba109c2c5b87b2a

diff --git a/afile b/afile
index 14be0d4..7c2160f 100644
--- a/afile
+++ b/afile

@@ -1 +1 @@
-hello2
+goodbye2

diff --git a/sub b/sub
deleted file mode 160000
index c17499a..0000000
--- a/sub
+++ /dev/null

@@ -1 +0,0 @@
-Subproject commit c17499ae216dfba9538ed7f94ba109c2c5b87b2a

- Orgad