t6019: avoid refname collision on case-insensitive systems

The criss-cross tests kept failing for me because of collisions of 'a'
with 'A' etc.  Prefix the lowercase refnames with an extra letter to
disambiguate.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6019-rev-list-ancestry-path.sh

@@ -75,33 +75,36 @@ test_expect_success 'rev-list --ancestry-patch D..M -- M.t' '
 # a   X
 #  \ / \
 #   c---cb
+#
+# All refnames prefixed with 'x' to avoid confusion with the tags
+# generated by test_commit on case-insensitive systems.
 test_expect_success 'setup criss-cross' '
 	mkdir criss-cross &&
 	(cd criss-cross &&
 	 git init &&
 	 test_commit A &&
-	 git checkout -b b master &&
+	 git checkout -b xb master &&
 	 test_commit B &&
-	 git checkout -b c master &&
+	 git checkout -b xc master &&
 	 test_commit C &&
-	 git checkout -b bc b -- &&
+	 git checkout -b xbc xb -- &&
-	 git merge c &&
+	 git merge xc &&
-	 git checkout -b cb c -- &&
+	 git checkout -b xcb xc -- &&
-	 git merge b &&
+	 git merge xb &&
 	 git checkout master)
 '
 
 # no commits in bc descend from cb
 test_expect_success 'criss-cross: rev-list --ancestry-path cb..bc' '
 	(cd criss-cross &&
-	 git rev-list --ancestry-path cb..bc > actual &&
+	 git rev-list --ancestry-path xcb..xbc > actual &&
 	 test -z "$(cat actual)")
 '
 
 # no commits in repository descend from cb
 test_expect_success 'criss-cross: rev-list --ancestry-path --all ^cb' '
 	(cd criss-cross &&
-	 git rev-list --ancestry-path --all ^cb > actual &&
+	 git rev-list --ancestry-path --all ^xcb > actual &&
 	 test -z "$(cat actual)")
 '