grep: adjust a redundant grep pattern type assignment

grep: adjust a redundant grep pattern type assignment

Adjust a now-redundant assignment to extended_regexp_option to make it
zero if grep.extendedRegexp is not set. This is always called right
after init_grep_defaults() which memsets the entire structure to 0, so
there's no need to set it again to zero.

However the reason for the if/else pattern is a holdover from[1] where
this was adjusted from a bitfield assignment to a boolean. Rather than
getting rid of the assignment to 0 in all cases, let's just use the
value returned by git_config_bool(), which is more idiomatic and in
sync with the rest of the boolean handling in this function.

This is a logical follow-up to my commit to remove redundant regflags
assignments[2]. This logic was originally introduced in [3], but as
explained in the former commit it's working around a pattern in our
code that no longer exists, and is now confusing as it leads the
reader to think that this needs to be flipped back & forth.

1. 84befcd0a4 ("grep: add a grep.patternType configuration setting",
   2012-08-03)
2. e0b9f8ae09 ("grep: remove redundant regflags assignments",
   2017-05-25)
3. b22520a37c ("grep: allow -E and -n to be turned on by default via
   configuration", 2011-03-30)

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