tests: fix unportable "\?" and "\+" regex syntax

Fix widely supported but non-POSIX basic regex syntax introduced in [1] and [2]. On GNU, NetBSD and FreeBSD the following works: $ echo xy >f $ grep 'xy\?' f; echo $? xy 0 The same goes for "\+". The "?" and "+" syntax is not in the BRE syntax, just in ERE, but on some implementations it can be invoked by prefixing the meta-operator with "\", but not on OpenBSD: $ uname -a OpenBSD obsd.my.domain 6.2 GENERIC#132 amd64 $ grep --version grep version 0.9 $ grep 'xy\?' f; echo $? 1 Let's fix this by moving to ERE syntax instead, where "?" and "+" are universally supported: $ grep -E 'xy?' f; echo $? xy 0 1. 2ed5c8e174 ("describe: setup working tree for --dirty", 2019-02-03) 2. c801170b0c ("t6120: test for describe with a bare repository", 2019-02-03) Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>