t5533: make it pass on case-sensitive filesystems

The newly-added test case wants to commit a file "c.t" (note the lower
case) when a previous test case already committed a file "C.t". This
confuses Git to the point that it thinks "c.t" was not staged when "git
add c.t" was called.

Simply make the naming of the test commits consistent with the previous
test cases: use upper-case, and advance in the alphabet.

This came up in local work to rebase the Windows-specific patches to the
current `next` branch. An identical fix was suggested by John Keeping.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5533-push-cas.sh

@@ -220,7 +220,7 @@ test_expect_success 'new branch already exists' '
 	(
 		cd src &&
 		git checkout -b branch master &&
-		test_commit c
+		test_commit F
 	) &&
 	(
 		cd dst &&