t5607: avoid using prerequisites to select algorithm

In this test, we currently use the SHA1 prerequisite to specify the
algorithm we're using to test, since SHA-256 bundles are always v3,
whereas SHA-1 bundles default to v2, and as a result the default output
differs.

However, this causes a problem if we run with GIT_TEST_FAIL_PREREQS set,
since that means that we'll unexpectedly fail the SHA1 prerequisite,
resulting in incorrect expected output.  Let's fix this by checking
against the built-in data called "algo", which tells us which algorithm
is in use.  This should work in any situation, making our test a little
more robust.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5607-clone-bundle.sh

@@ -91,7 +91,8 @@ test_expect_success 'ridiculously long subject in boundary' '
 
 	git fetch long-subject-bundle.bdl &&
 
-	if ! test_have_prereq SHA1
+	algo=$(test_oid algo) &&
+	if test "$algo" != sha1
 	then
 		echo "@object-format=sha256"
 	fi >expect &&
@@ -100,7 +101,7 @@ test_expect_success 'ridiculously long subject in boundary' '
 	$(git rev-parse HEAD) HEAD
 	EOF
 
-	if test_have_prereq SHA1
+	if test "$algo" = sha1
 	then
 		head -n 3 long-subject-bundle.bdl
 	else