grep.c: remove "extended" in favor of "pattern_expression", fix segfault

grep.c: remove "extended" in favor of "pattern_expression", fix segfault

Since 79d3696cfb4 (git-grep: boolean expression on pattern matching.,
2006-06-30) the "pattern_expression" member has been used for complex
queries (AND/OR...), with "pattern_list" being used for the simple OR
queries. Since then we've used both "pattern_expression" and its
associated boolean "extended" member to see if we have a complex
expression.

Since f41fb662f57 (revisions API: have release_revisions() release
"grep_filter", 2022-04-13) we've had a subtle bug relating to that: If
we supplied options that were only used for "complex queries", but
didn't supply the query itself we'd set "opt->extended", but would
have a NULL "pattern_expression". As a result these would segfault as
we tried to call "free_grep_patterns()" from "release_revisions()":

	git -P log -1 --invert-grep
	git -P log -1 --all-match

The root cause of this is that we were conflating the state management
we needed in "compile_grep_patterns()" itself with whether or not we
had an "opt->pattern_expression" later on.

In this cases as we're going through "compile_grep_patterns()" we have
no "opt->pattern_list" but have "opt->no_body_match" or
"opt->all_match". So we'd set "opt->extended = 1", but not "return" on
"opt->extended" as that's an "else if" in the same "if" statement.

That behavior is intentional and required, as the common case is that
we have an "opt->pattern_list" that we're about to parse into the
"opt->pattern_expression".

But we don't need to keep track of this "extended" flag beyond the
state management in compile_grep_patterns() itself. It needs it, but
once we're out of that function we can rely on
"opt->pattern_expression" being non-NULL instead for using these
extended patterns.

As 79d3696cfb4 itself shows we've assumed that there's a one-to-one
mapping between the two since the very beginning. I.e. "match_line()"
would check "opt->extended" to see if it should call "match_expr()",
and the first thing we do in that function is assume that we have a
"opt->pattern_expression". We'd then call "match_expr_eval()", which
would have died if that "opt->pattern_expression" was NULL.

The "die" was added in c922b01f54c (grep: fix segfault when "git grep
'('" is given, 2009-04-27), and can now be removed as it's now clearly
unreachable. We still do the right thing in the case that prompted
that fix:

	git grep '('
	fatal: unmatched parenthesis

Arguably neither the "--invert-grep" option added in [1] nor the
earlier "--all-match" option added in [2] were intended to be used
stand-alone, and another approach[3] would be to error out in those
cases. But since we've been treating them as a NOOP when given without
--grep for a long time let's keep doing that.

We could also return in "free_pattern_expr()" if the argument is
non-NULL, as an alternative fix for this segfault does [4]. That would
be more elegant in making the "free_*()" function behave like
"free()", but it would also remove a sanity check: The
"free_pattern_expr()" function calls itself recursively, and only the
top-level is allowed to be NULL, let's not conflate those two
conditions.

1. 22dfa8a23de (log: teach --invert-grep option, 2015-01-12)
2. 0ab7befa31d (grep --all-match, 2006-09-27)
3. https://lore.kernel.org/git/patch-1.1-f4b90799fce-20221010T165711Z-avarab@gmail.com/
4. http://lore.kernel.org/git/7e094882c2a71894416089f894557a9eae07e8f8.1665423686.git.me@ttaylorr.com

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