grep: remove regflags from the public grep_opt API

Refactor calls to the grep machinery to always pass opt.ignore_case & opt.extended_regexp_option instead of setting the equivalent regflags bits. The bug fixed when making -i work with -P in commit 9e3cbc59d5 ("log: make --regexp-ignore-case work with --perl-regexp", 2017-05-20) was really just plastering over the code smell which this change fixes. The reason for adding the extensive commentary here is that I discovered some subtle complexity in implementing this that really should be called out explicitly to future readers. Before this change we'd rely on the difference between `extended_regexp_option` and `regflags` to serve as a membrane between our preliminary parsing of grep.extendedRegexp and grep.patternType, and what we decided to do internally. Now that those two are the same thing, it's necessary to unset `extended_regexp_option` just before we commit in cases where both of those config variables are set. See 84befcd0a4 ("grep: add a grep.patternType configuration setting", 2012-08-03) for the code and documentation related to that. The explanation of why the if/else branches in grep_commit_pattern_type() are ordered the way they are exists in that commit message, but I think it's worth calling this subtlety out explicitly with a comment for future readers. Even though grep_commit_pattern_type() is the only caller of grep_set_pattern_type_option() it's simpler to reset the extended_regexp_option flag in the latter, since 2/3 branches in the former would otherwise need to reset it, this way we can do it in one place. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>