grep tests: add missing "grep.patternType" config tests

Extend the grep tests to assert that setting
"grep.patternType=extended" followed by "grep.patternType=default"
will behave as if "--basic-regexp" was provided, and not as
"--extended-regexp". In a subsequent commit we'll need to treat
"grep.patternType=default" as a special-case, but let's make sure we
ignore it if it's being set to "default" following an earlier
non-"default" "grep.patternType" setting.

Let's also test what happens when we have a sequence of "extended"
followed by "default" and "fixed". In that case the "fixed" should
prevail, as well as tests to check that a "grep.extendedRegexp=true"
followed by a "grep.extendedRegexp=false" behaves as though
"grep.extendedRegexp" wasn't provided.

See [1] for the source of some of these tests, and their
initial (pseudocode) implementation, and [2] for a later discussion
about a breakage due to missing testing (which had been noted in [1]
all along).

1. https://lore.kernel.org/git/xmqqv8zf6j86.fsf@gitster.g/
2. https://lore.kernel.org/git/xmqqpmoczwtu.fsf@gitster.g/

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

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