reachable: mark index blobs as SEEN

When we mark all reachable objects for pruning, that
includes blobs mentioned by the index. However, we do not
mark these with the SEEN flag, as we do for objects that we
find by traversing (we also do not add them to the pending
list, but that is because there is nothing further to
traverse with them).

This doesn't cause any problems with prune, because it
checks only that the object exists in the global object
hash, and not its flags. However, let's mark these objects
to be consistent and avoid any later surprises.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

reachable.c

@@ -55,6 +55,8 @@ static void add_cache_refs(struct rev_info *revs)
 
 	read_cache();
 	for (i = 0; i < active_nr; i++) {
+		struct blob *blob;
+
 		/*
 		 * The index can contain blobs and GITLINKs, GITLINKs are hashes
 		 * that don't actually point to objects in the repository, it's
@@ -65,7 +67,10 @@ static void add_cache_refs(struct rev_info *revs)
 		if (S_ISGITLINK(active_cache[i]->ce_mode))
 			continue;
 
-		lookup_blob(active_cache[i]->sha1);
+		blob = lookup_blob(active_cache[i]->sha1);
+		if (blob)
+			blob->object.flags |= SEEN;
+
 		/*
 		 * We could add the blobs to the pending list, but quite
 		 * frankly, we don't care. Once we've looked them up, and