grep: fix exit status if external_grep() punts

If external_grep() is called and punts, grep_cache() mistakenly reported a
hit, even if there were none.  The bug can be triggered by calling "git
grep --no-color" from a subdirectory.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-grep.c

@@ -448,6 +448,7 @@ static int grep_cache(struct grep_opt *opt, const char **paths, int cached)
 		hit = external_grep(opt, paths, cached);
 		if (hit >= 0)
 			return hit;
+		hit = 0;
 	}
 #endif