Make --files-from only read the objects specified and don't scan directories

Before this change using --files-from would scan all the directories
that the files could possibly be in causing rclone to do more work
that was necessary.

After this change, rclone constructs an in memory tree using the
--fast-list mechanism but from all of the files in the --files-from
list and without scanning any directories.

Any objects that are not found in the --files-from list are ignored
silently.

This mechanism is used for sync/copy/move (march) and all of the
listing commands ls/lsf/md5sum/etc (walk).

fs/sync/sync_test.go

@@ -79,6 +79,35 @@ func TestCopyWithDepth(t *testing.T) {
 	fstest.CheckItems(t, r.Fremote, file2)
 }
 
+// Test copy with files from
+func TestCopyWithFilesFrom(t *testing.T) {
+	r := fstest.NewRun(t)
+	defer r.Finalise()
+	file1 := r.WriteFile("potato2", "hello world", t1)
+	file2 := r.WriteFile("hello world2", "hello world2", t2)
+
+	// Set the --files-from equivalent
+	f, err := filter.NewFilter(nil)
+	require.NoError(t, err)
+	require.NoError(t, f.AddFile("potato2"))
+	require.NoError(t, f.AddFile("notfound"))
+
+	// Monkey patch the active filter
+	oldFilter := filter.Active
+	filter.Active = f
+	unpatch := func() {
+		filter.Active = oldFilter
+	}
+	defer unpatch()
+
+	err = CopyDir(r.Fremote, r.Flocal)
+	require.NoError(t, err)
+	unpatch()
+
+	fstest.CheckItems(t, r.Flocal, file1, file2)
+	fstest.CheckItems(t, r.Fremote, file1)
+}
+
 // Test copy empty directories
 func TestCopyEmptyDirectories(t *testing.T) {
 	r := fstest.NewRun(t)