grep: remove grep_opt argument from match_expr_eval()

The only use of the struct grep_opt argument of match_expr_eval()
is to pass the option word_regexp to match_one_pattern().  By adding
a pattern flag for it we can reduce the number of function arguments
of these two functions, as a cleanup and preparation for adding more
in the next patch.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.h

@@ -31,6 +31,7 @@ struct grep_pat {
 	enum grep_header_field field;
 	regex_t regexp;
 	unsigned fixed:1;
+	unsigned word_regexp:1;
 };
 
 enum grep_expr_node {