t6300: avoid creating refs/heads/HEAD

In one test, we use "git checkout --orphan HEAD" to create
an unborn branch. Confusingly, the resulting branch is named
"refs/heads/HEAD". The original probably meant something
like:

  git checkout --orphan orphaned-branch HEAD

Let's just use "orphaned-branch" here to make this less
confusing. Putting HEAD in the second argument is already
implied.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

t/t6300-for-each-ref.sh

@@ -558,7 +558,7 @@ test_expect_success 'do not dereference NULL upon %(HEAD) on unborn branch' '
 	test_when_finished "git checkout master" &&
 	git for-each-ref --format="%(HEAD) %(refname:short)" refs/heads/ >actual &&
 	sed -e "s/^\* /  /" actual >expect &&
-	git checkout --orphan HEAD &&
+	git checkout --orphan orphaned-branch &&
 	git for-each-ref --format="%(HEAD) %(refname:short)" refs/heads/ >actual &&
 	test_cmp expect actual
 '