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

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>