grep: add option to show whole function as context

Add a new option, -W, to show the whole surrounding function of a match.

It uses the same regular expressions as -p and diff to find the beginning
of sections.

Currently it will not display comments in front of a function, but those
that are following one.  Despite this shortcoming it is already useful,
e.g. to simply see a more complete applicable context or to extract whole
functions.

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

grep.h

@@ -98,6 +98,7 @@ struct grep_opt {
 	int color;
 	int max_depth;
 	int funcname;
+	int funcbody;
 	char color_context[COLOR_MAXLEN];
 	char color_filename[COLOR_MAXLEN];
 	char color_function[COLOR_MAXLEN];