grep: amend submodule recursion test for regex engine testing

grep: amend submodule recursion test for regex engine testing

Amend the submodule recursion test to prepare it for subsequent tests
of whether it passes along the grep.patternType to the submodule
greps.

This is the result of searching & replacing:

    foobar -> (1|2)d(3|4)
    foo    -> (1|2)
    bar    -> (3|4)

Currently there's no tests for whether e.g. -P or -E is correctly
passed along, tests for that will be added in a follow-up change, but
first add content to the tests which will match differently under
different regex engines.

Reuse the pattern established in an earlier commit of mine in this
series ("log: add exhaustive tests for pattern style options &
config", 2017-04-07). The pattern "(.|.)[\d]" will match this content
differently under fixed/basic/extended & perl.

This test code was originally added in commit 0281e487fd ("grep:
optionally recurse into submodules", 2016-12-16).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>