Fixed a bug that didn't preserve the version bit when copying old snapshots

The version bit should not be set to 1 when encoding a snapshot.  Instead,
it must be set to 1 on snapshot creation.

To correctly process old snapshots encoded incorrectly with version bit set
to 1, the first byte of the encoded file list is also checked.  If the first
byte is `[`, then it must be an old snapshot, since the file list in the new
snapshot format always starts with a string encoded in msgpack, the first
byte of which can't be `[`.

src/duplicacy_snapshotmanager.go

@@ -249,17 +249,27 @@ func (manager *SnapshotManager) DownloadSnapshot(snapshotID string, revision int
 // the memory before passing them to the json unmarshaller.
 type sequenceReader struct {
 	sequence   []string
-	buffer     *bytes.Buffer
+	buffer     *bytes.Reader
 	index      int
 	refillFunc func(hash string) []byte
 }
 
+func NewSequenceReader(sequence []string, refillFunc func(hash string) []byte) *sequenceReader {
+	newData := refillFunc(sequence[0])
+	return &sequenceReader{
+		sequence:   sequence,
+		buffer:     bytes.NewReader(newData),
+		index:      1,
+		refillFunc: refillFunc,
+	}
+}
+
 // Read reads a new chunk using the refill function when there is no more data in the buffer
 func (reader *sequenceReader) Read(data []byte) (n int, err error) {
-	if len(reader.buffer.Bytes()) == 0 {
+	if reader.buffer.Len() == 0 {
 		if reader.index < len(reader.sequence) {
 			newData := reader.refillFunc(reader.sequence[reader.index])
-			reader.buffer.Write(newData)
+			reader.buffer = bytes.NewReader(newData)
 			reader.index++
 		} else {
 			return 0, io.EOF
@@ -269,6 +279,16 @@ func (reader *sequenceReader) Read(data []byte) (n int, err error) {
 	return reader.buffer.Read(data)
 }
 
+func (reader *sequenceReader) GetFirstByte() byte {
+	b, err := reader.buffer.ReadByte()
+	reader.buffer.UnreadByte()
+	if err != nil {
+		return 0
+	} else {
+		return b
+	}
+}
+
 func (manager *SnapshotManager) CreateChunkOperator(resurrect bool, rewriteChunks bool, threads int, allowFailures bool) {
 	if manager.chunkOperator == nil {
 		manager.chunkOperator = CreateChunkOperator(manager.config, manager.storage, manager.snapshotCache, resurrect, rewriteChunks, threads, allowFailures)