t7600: don't ignore "rev-parse" exit code in helper

Change the verify_mergeheads() helper the check the exit code of "git
rev-parse".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7600-merge.sh

@@ -105,7 +105,7 @@ verify_mergeheads () {
 	test_write_lines "$@" >mergehead.expected &&
 	while read sha1 rest
 	do
-		git rev-parse $sha1
+		git rev-parse $sha1 || return 1
 	done <.git/MERGE_HEAD >mergehead.actual &&
 	test_cmp mergehead.expected mergehead.actual
 }