t5520: avoid alternation in grep's BRE (not POSIX)

Instead of using a BRE, that broke tests 30-32, 37-39, 42 at least with
OpenBSD 6.7; use a simpler ERE.

Fixes: d9f15d37f1 (pull: pass --autostash to merge, 2020-04-07)
Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5520-pull.sh

@@ -28,7 +28,7 @@ test_pull_autostash_fail () {
 	echo dirty >new_file &&
 	git add new_file &&
 	test_must_fail git pull "$@" . copy 2>err &&
-	test_i18ngrep "\(uncommitted changes.\)\|\(overwritten by merge:\)" err
+	test_i18ngrep -E "uncommitted changes.|overwritten by merge:" err
 }
 
 test_expect_success setup '