t1512: skip test if not using SHA-1

This test relies on objects with colliding short names which are
necessarily dependent on the hash used.  Skip the test if we're not
using SHA-1.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t1512-rev-parse-disambiguation.sh b/t/t1512-rev-parse-disambiguation.sh
index 711704b..6537f30 100755
--- a/t/t1512-rev-parse-disambiguation.sh
+++ b/t/t1512-rev-parse-disambiguation.sh
@@ -22,6 +22,12 @@
 
 . ./test-lib.sh
 
+if ! test_have_prereq SHA1
+then
+	skip_all='not using SHA-1 for objects'
+	test_done
+fi
+
 test_expect_success 'blob and tree' '
 	test_tick &&
 	(