t5520: don't put git in upstream of pipe

Before, if the invocation of git failed, it would be masked by the pipe
since only the return code of the last element of a pipe is used.
Rewrite the test to put the git command on its own line so its return
code is not masked.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5520-pull.sh

@@ -668,7 +668,8 @@ test_expect_success 'pull --rebase fails on corrupt HEAD' '
 	(
 		cd corrupt &&
 		test_commit one &&
-		obj=$(git rev-parse --verify HEAD | sed "s#^..#&/#") &&
+		git rev-parse --verify HEAD >head &&
+		obj=$(sed "s#^..#&/#" head) &&
 		rm -f .git/objects/$obj &&
 		test_must_fail git pull --rebase
 	)