log tests: check if grep_config() is called by "log"-like cmds

log tests: check if grep_config() is called by "log"-like cmds

Extend the tests added in my 9df46763ef1 (log: add exhaustive tests
for pattern style options & config, 2017-05-20) to check not only
whether "git log" handles "grep.patternType", but also "git show"
etc.

It's sufficient to check whether we match a "fixed" or a "basic" regex
here to see if these codepaths correctly invoked grep_config(). We
don't need to check the details of their regular expression matching
as the "log" test does.

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