Fix t3400 on a case insensitive file system

On a case insenstive file system x is the same as X.
When a file named "X" exist, e.g. ">x && git add x" does not work as expected, use file name "Y" instead.

---
 t/t3400-rebase.sh | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

index 5ed4012..2eebd22 100755
@@ -259,11 +259,11 @@ test_expect_success 'rebase duplicated commit with
--keep-empty' '
 	git reset --hard &&
 	git checkout master &&
 -	>x && git add x && git commit x -mx &&
-	echo x >x && git commit x -mx1 &&
+	>Y && git add Y && git commit Y -mY &&
+	echo Y >Y && git commit Y -mY1 &&
  	git checkout -b duplicated HEAD~ &&
-	echo x >x && git commit x -mx2 &&
+	echo Y >Y && git commit Y -mY2 &&
 	git rebase --keep-empty master
 '
 -- 2.6.2.403.g6abe3ff.dirty

It is a right thing to fix this, but do you have to use capital 'Y' here?

I see both uppercase X and Y are used in the earlier tests in this script, and it may be that that earlier Y, unlike earlier X, does not interfere on your case insensitive system right now, but using a single letter 'Y' smells like a future breakage waiting to happen. Wouldn't it make more sense to name this a lot more distinctively and uniquely?

In any case, I think this is only in the topic that has been listed as "Will discard", so I won't worry about it myself for now--if the topic will be resurrected in the future, reviewers should keep an eye to prevent it from adding this bug.

Thanks.