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>