bisync: generate final listing from sync results, not relisting -- fixes #5676

Before this change, if there were changes to sync, bisync listed each path
twice: once before the sync and once after. The second listing caused quite
a lot of problems, in addition to making each run much slower and more
expensive. A serious side-effect was that file changes could slip through
undetected, if they happened to occur while a sync was running (between the
first and second listing snapshots.)

After this change, the second listing is eliminated by getting the underlying
sync operation to report back a list of what it changed. Not only is this more
efficient, but also much more robust to concurrent modifications. It should no
longer be necessary to avoid make changes while it's running -- bisync will
simply learn about those changes next time and handle them on the next run.
Additionally, this also makes --check-sync usable again.

For further discussion, see:
https://forum.rclone.org/t/bisync-bugs-and-feature-requests/37636#:~:text=5.%20Final%20listings%20should%20be%20created%20from%20initial%20snapshot%20%2B%20deltas%2C%20not%20full%20re%2Dscans%2C%20to%20avoid%20errors%20if%20files%20changed%20during%20sync

cmd/bisync/bisync_test.go

@@ -641,6 +641,8 @@ func (b *bisyncTest) runBisync(ctx context.Context, args []string) (err error) {
 		case "subdir":
 			fs1 = addSubdir(b.path1, val)
 			fs2 = addSubdir(b.path2, val)
+		case "ignore-listing-checksum":
+			opt.IgnoreListingChecksum = true
 		default:
 			return fmt.Errorf("invalid bisync option %q", arg)
 		}