grep -p: support user defined regular expressions

Respect the userdiff attributes and config settings when looking for
lines with function definitions in git grep -p.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/grep.h b/grep.h
index 3f75e3a..f00db0e 100644
--- a/grep.h
+++ b/grep.h
@@ -87,6 +87,7 @@
 	unsigned post_context;
 	unsigned last_shown;
 	int show_hunk_mark;
+	void *priv;
 };
 
 extern void append_grep_pattern(struct grep_opt *opt, const char *pat, const char *origin, int no, enum grep_pat_token t);