storage: add copyDir

we changed listDir in c44863a9bb to list
a directory . this broke copy for files since a '/' is added when listing
the file.

src/backupformat/tgz.js

@@ -271,6 +271,15 @@ async function upload(backupTarget, remotePath, dataLayout, progressCallback) {
     });
 }
 
+async function copy(backupTarget, fromPath, toPath, progressCallback) {
+    assert.strictEqual(typeof backupTarget, 'object');
+    assert.strictEqual(typeof fromPath, 'string');
+    assert.strictEqual(typeof toPath, 'string');
+    assert.strictEqual(typeof progressCallback, 'function');
+
+    await backupTargets.storageApi(backupTarget).copy(backupTarget.config, fromPath, toPath, progressCallback);
+}
+
 async function verify(backupTarget, remotePath, integrityMap, progressCallback) {
     assert.strictEqual(typeof backupTarget, 'object');
     assert.strictEqual(typeof remotePath, 'string');
@@ -340,6 +349,7 @@ exports = module.exports = {
     upload,
     verify,
     getFileExtension,
+    copy,
 
     // exported for testing
     _EnsureFileSizeStream: EnsureFileSizeStream