grep: remove grep_opt argument from match_expr_eval()

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>