Problems with extra files on git filter-branch when core.ignorecase is true

Today when trying to change the contents of a repository with git
filter-branch, a tool that was run on a part of the working tree for
each version reported an extra file that couldn't be accounted for.
After some testing and a small discussion on the irc channel, I was
able to come up with the following scenario to reproduce the issue.
The test was performed under Ubuntu with Git 1.7.0.4 on a case
sensitive file system:

~$ mkdir gittest
~$ cd gittest
~/gittest$ git init
Initialized empty Git repository in /home/user/gittest/.git/
~/gittest$ git config core.ignorecase true
~/gittest$ echo 'foo' > testfile.txt
~/gittest$ git add testfile.txt
~/gittest$ git commit -m "initial commit"
[master (root-commit) 794888c] initial commit
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 testfile.txt
~/gittest$ git rm testfile.txt
rm 'testfile.txt'
~/gittest$ echo 'bar' > testFile.txt
~/gittest$ git add testFile.txt
~/gittest$ git commit -m "additional commit"
[master a669956] additional commit
 2 files changed, 1 insertions(+), 1 deletions(-)
 create mode 100644 testFile.txt
 delete mode 100644 testfile.txt
~/gittest$ git filter-branch --tree-filter 'ls' master
Rewrite 794888cdbe107aa7408cc5ed4132dcd3bc2753c8 (1/2)testfile.txt
Rewrite a66995628af5710828a6a59628553938bc30ee37 (2/2)testfile.txt  testFile.txt

WARNING: Ref 'refs/heads/master' is unchanged

In the second rewrite line, I would only expect testFile.txt to
appear, regardless of the core.ignorecase setting. On a case
insensitive filesystem the above example works correctly.

It might seem strange to set core.ignorecase to true on a case
sensitive filesystem, however, this issue also appears when using a
(case sensitive) tmpfs temp directory to filter a case insensitive
repository.

Thanks in advance for your help.

Simeon