Implement --no-traverse flag to stop copy traversing the destination remote.

Refactor sync/copy/move
  * Don't load the src listing unless doing a sync and --delete-before
  * Don't load the dst listing if doing copy/move and --no-traverse is set

`rclone --no-traverse copy src dst` now won't load either of the
listings into memory so will use the minimum amount of memory.

This change will reduce the amount of memory rclone uses dramatically
too as in normal operations (copy without --notraverse or sync) as it
no longer loads the source file listing into memory at all.

Fixes #8
Fixes #544
Fixes #546

fs/config.go

@@ -88,6 +88,7 @@ var (
 	dedupeMode      = pflag.StringP("dedupe-mode", "", "interactive", "Dedupe mode interactive|skip|first|newest|oldest|rename.")
 	maxDepth        = pflag.IntP("max-depth", "", -1, "If set limits the recursion depth to this.")
 	ignoreSize      = pflag.BoolP("ignore-size", "", false, "Ignore size when skipping use mod-time or checksum.")
+	noTraverse      = pflag.BoolP("no-traverse", "", false, "Don't traverse destination file system on copy.")
 	bwLimit         SizeSuffix
 
 	// Key to use for password en/decryption.
@@ -223,6 +224,7 @@ type ConfigInfo struct {
 	DedupeMode         DeduplicateMode
 	MaxDepth           int
 	IgnoreSize         bool
+	NoTraverse         bool
 }
 
 // Transport returns an http.RoundTripper with the correct timeouts
@@ -327,6 +329,7 @@ func LoadConfig() {
 	Config.NoGzip = *noGzip
 	Config.MaxDepth = *maxDepth
 	Config.IgnoreSize = *ignoreSize
+	Config.NoTraverse = *noTraverse
 
 	ConfigPath = *configFile