t4208: abstract away SHA-1-specific constants

Adjust the test so that it computes variables for object IDs instead of
using hard-coded hashes.

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

t/t4208-log-magic-pathspec.sh

@@ -45,8 +45,9 @@ test_expect_success 'git log -- :' '
 '
 
 test_expect_success 'git log HEAD -- :/' '
+	initial=$(git rev-parse --short HEAD^) &&
 	cat >expected <<-EOF &&
-	24b24cf initial
+	$initial initial
 	EOF
 	(cd sub && git log --oneline HEAD -- :/ >../actual) &&
 	test_cmp expected actual