grep -P: Fix matching ^ and $

When "git grep" is run with -P/--perl-regexp, it doesn't match ^ and $ at
the beginning/end of the line.  This is because PCRE normally matches ^
and $ at the beginning/end of the whole text, not for each line, and "git
grep" passes a large chunk of text (possibly containing many lines) to
pcre_exec() and then splits the text into lines.

This makes "git grep -P" behave differently from "git grep -E" and also
from "grep -P" and "pcregrep":

	$ cat file
	a
	 b
	$ git grep --no-index -P '^ ' file
	$ git grep --no-index -E '^ ' file
	file: b
	$ grep -c -P '^ ' file
	 b
	$ pcregrep -c '^ ' file
	 b

Reported-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Michał Kiedrowicz <michal.kiedrowicz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -79,7 +79,7 @@ static void compile_pcre_regexp(struct grep_pat *p, const struct grep_opt *opt)
 {
 	const char *error;
 	int erroffset;
-	int options = 0;
+	int options = PCRE_MULTILINE;
 
 	if (opt->ignore_case)
 		options |= PCRE_CASELESS;