t0410: keep git's exit code in test_grep calls

The 'rev-list stops traversal at missing and promised tree' test passes
"$(git -C repo rev-parse ...)" directly to test_grep. t/README says
not to use command substitution in a way that discards git's exit
code: the && chain only sees test_grep's exit code, so a failure in
git is not caught there.

Assign the rev-parse output to variables first and pass those to
test_grep instead.

Signed-off-by: Dorine Tipo <dorine.a.tipo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: MissTipo

t/t0410-partial-clone.sh

@@ -413,9 +413,13 @@ test_expect_success 'rev-list stops traversal at missing and promised tree' '
 	git -C repo config core.repositoryformatversion 1 &&
 	git -C repo config extensions.partialclone "arbitrary string" &&
 	git -C repo rev-list --exclude-promisor-objects --objects HEAD >out &&
-	test_grep $(git -C repo rev-parse foo) out &&
+
+	FOO=$(git -C repo rev-parse foo) &&
+	HEAD_OID=$(git -C repo rev-parse HEAD) &&
+
+	test_grep "$FOO" out &&
 	test_grep ! $TREE out &&
-	test_grep $(git -C repo rev-parse HEAD) out &&
+	test_grep "$HEAD_OID" out &&
 	test_grep ! $TREE2 out
 '