dir.c: support marking some patterns already matched

Given path "a" and the pattern "a", it's matched. But if we throw path
"a/b" to pattern "a", the code fails to realize that if "a" matches
"a" then "a/b" should also be matched.

When the pattern is matched the first time, we can mark it "sticky", so
that all files and dirs inside the matched path also matches. This is a
simpler solution than modify all match scenarios to fix that.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.h

@@ -4,6 +4,7 @@
 /* See Documentation/technical/api-directory-listing.txt */
 
 #include "strbuf.h"
+#include "string-list.h"
 
 struct dir_entry {
 	unsigned int len;
@@ -34,6 +35,8 @@ struct exclude {
 	 * and from -1 decrementing for patterns from CLI args.
 	 */
 	int srcpos;
+
+	struct string_list sticky_paths;
 };
 
 /*