grep: create a "is_fixed" member in "grep_pat"

This change paves the way for later using this value the regex compile
functions themselves.

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

grep.h

@@ -88,6 +88,7 @@ struct grep_pat {
 	pcre2_compile_context *pcre2_compile_context;
 	uint32_t pcre2_jit_on;
 	unsigned fixed:1;
+	unsigned is_fixed:1;
 	unsigned ignore_case:1;
 	unsigned word_regexp:1;
 };