Cygwin: problem with renaming and case

[Frank <streamlake@tiscali.it>]

Hi,
Don't know exactly if this is a bug or a feature or something in the 
middle, but I have a lot of problems while changing just the casing of 
file names and using git mv und cygwin. Here's a test case:

mkdir testrename
cd testrename
git init
echo "AAA" >aaa.txt
echo "BBB" >bbb.txt
git add aaa.txt
git add bbb.txt
git commit -m "First commit"
git checkout -b new_branch
git mv aaa.txt ccc.txt
git commit -a -m "Moved file"
echo "NEW AAA" >Aaa.txt
git add Aaa.txt
git commit -m "Added Aaa"
#aaa.txt exists in master, Aaa.txt in new_branch
git checkout master

Last command gives: "fatal: Untracked working tree file 'aaa.txt' would 
be overwritten by merge".
I know I can use git checkout -f but the problem returns while others do 
merging/pulling from my repo, etc.
Thanks,
Frank