t3103: abstract away SHA-1-specific constants

Adjust the test so that it uses variables and command substitution for
trees instead of hard-coded hashes.  This also has the benefit of making
it more obvious how the test works.

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

t/t3103-ls-tree-misc.sh

@@ -17,7 +17,8 @@ test_expect_success 'setup' '
 '
 
 test_expect_success 'ls-tree fails with non-zero exit code on broken tree' '
-	rm -f .git/objects/5f/cffbd6e4c5c5b8d81f5e9314b20e338e3ffff5 &&
+	tree=$(git rev-parse HEAD:a) &&
+	rm -f .git/objects/$(echo $tree | sed -e "s,^\(..\),\1/,") &&
 	test_must_fail git ls-tree -r HEAD
 '