t6050: redirect expected error output to a file

Otherwise the error from `git rev-parse` is uselessly
polluting the debug output.

Redirecting to a file, instead of /dev/null, makes it
possible to check that we got the error we expected, so
let's check that too.

Reviewed-by: Taylor Blau <me@ttaylorr.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6050-replace.sh

@@ -40,7 +40,8 @@ commit_peeling_shows_parents ()
 	test "$_found" = "$_parent" || return 1
 	_parent_number=$(( $_parent_number + 1 ))
     done &&
-    test_must_fail git rev-parse --verify $_commit^$_parent_number
+    test_must_fail git rev-parse --verify $_commit^$_parent_number 2>err &&
+    test_i18ngrep "Needed a single revision" err
 }
 
 commit_has_parents ()