grep: add pmatch and eflags arguments to match_one_pattern()

grep: add pmatch and eflags arguments to match_one_pattern()

Push pmatch and eflags to the callers of match_one_pattern(), which
allows them to specify regex execution flags and to get the location
of a match.

Since we only use the first element of the matches array and aren't
interested in submatches, no provision is made for callers to
provide a larger array.

eflags are ignored for fixed patterns, but that's OK, since they
only have a meaning in connection with regular expressions
containing ^ or $.

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