fix clang -Wtautological-compare with unsigned enum

Create a GREP_HEADER_FIELD_MIN so we can check that the field value is
sane and silence the clang warning.

Clang warning happens because the enum is unsigned (this is
implementation-defined, and there is no negative fields) and the check
is then tautological.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: John Keeping <john@keeping.me.uk>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -625,7 +625,8 @@ static struct grep_expr *prep_header_patterns(struct grep_opt *opt)
 	for (p = opt->header_list; p; p = p->next) {
 		if (p->token != GREP_PATTERN_HEAD)
 			die("bug: a non-header pattern in grep header list.");
-		if (p->field < 0 || GREP_HEADER_FIELD_MAX <= p->field)
+		if (p->field < GREP_HEADER_FIELD_MIN ||
+		    GREP_HEADER_FIELD_MAX <= p->field)
 			die("bug: unknown header field %d", p->field);
 		compile_regexp(p, opt);
 	}

grep.h

@@ -28,7 +28,8 @@ enum grep_context {
 };
 
 enum grep_header_field {
-	GREP_HEADER_AUTHOR = 0,
+	GREP_HEADER_FIELD_MIN = 0,
+	GREP_HEADER_AUTHOR = GREP_HEADER_FIELD_MIN,
 	GREP_HEADER_COMMITTER,
 	GREP_HEADER_REFLOG,