t6022: don't run 'git merge' upstream of a pipe

The primary purpose of 't6022-merge-rename.sh' is to test 'git merge',
but one of the tests runs it upstream of a pipe, hiding its exit code.
Consequently, the test could continue even if 'git merge' exited with
error.

Use an intermediate file between 'git merge' and 'test_i18ngrep' to
catch a potential failure of the former.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6022-merge-rename.sh

@@ -242,10 +242,12 @@ test_expect_success 'merge of identical changes in a renamed file' '
 	rm -f A M N &&
 	git reset --hard &&
 	git checkout change+rename &&
-	GIT_MERGE_VERBOSITY=3 git merge change | test_i18ngrep "^Skipped B" &&
+	GIT_MERGE_VERBOSITY=3 git merge change >out &&
+	test_i18ngrep "^Skipped B" out &&
 	git reset --hard HEAD^ &&
 	git checkout change &&
-	GIT_MERGE_VERBOSITY=3 git merge change+rename | test_i18ngrep "^Skipped B"
+	GIT_MERGE_VERBOSITY=3 git merge change+rename >out &&
+	test_i18ngrep "^Skipped B" out
 '
 
 test_expect_success 'setup for rename + d/f conflicts' '