Update grep internal for grepping only in head/body

This further updates the built-in grep engine so that we can say
something like "this pattern should match only in head".  This
can be used to simplify grepping in the log messages.

Signed-off-by: Junio C Hamano <junkio@cox.net>

grep.h

@@ -3,6 +3,8 @@
 
 enum grep_pat_token {
 	GREP_PATTERN,
+	GREP_PATTERN_HEAD,
+	GREP_PATTERN_BODY,
 	GREP_AND,
 	GREP_OPEN_PAREN,
 	GREP_CLOSE_PAREN,
@@ -10,6 +12,11 @@ enum grep_pat_token {
 	GREP_OR,
 };
 
+enum grep_context {
+	GREP_CONTEXT_HEAD,
+	GREP_CONTEXT_BODY,
+};
+
 struct grep_pat {
 	struct grep_pat *next;
 	const char *origin;