unpack-trees: hash less in cone mode

The sparse-checkout feature in "cone mode" can use the fact that
the recursive patterns are "connected" to the root via parent
patterns to decide if a directory is entirely contained in the
sparse-checkout or entirely removed.

In these cases, we can skip hashing the paths within those
directories and simply set the skipworktree bit to the correct
value.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -1270,7 +1270,7 @@ enum pattern_match_result path_matches_pattern_list(
 
 	if (hashmap_contains_path(&pl->recursive_hashmap,
 				  &parent_pathname)) {
-		result = MATCHED;
+		result = MATCHED_RECURSIVE;
 		goto done;
 	}
 
@@ -1292,7 +1292,7 @@ enum pattern_match_result path_matches_pattern_list(
 	if (hashmap_contains_parent(&pl->recursive_hashmap,
 				    pathname,
 				    &parent_pathname))
-		result = MATCHED;
+		result = MATCHED_RECURSIVE;
 
 done:
 	strbuf_release(&parent_pathname);