SFTPStream: add fileSize option for fast* methods

Supplying a valid file size will skip fstat/stat calls.

lib/sftp.js

@@ -995,6 +995,7 @@ function fastXfer(src, dst, srcPath, dstPath, opts, cb) {
   //var preserve = false;
   var onstep;
   var mode;
+  var fileSize;
 
   if (typeof opts === 'function') {
     cb = opts;
@@ -1007,6 +1008,10 @@ function fastXfer(src, dst, srcPath, dstPath, opts, cb) {
         && opts.chunkSize > 0
         && !isNaN(opts.chunkSize))
       chunkSize = opts.chunkSize;
+    if (typeof opts.fileSize === 'number'
+        && opts.fileSize > 0
+        && !isNaN(opts.fileSize))
+      fileSize = opts.fileSize;
     if (typeof opts.step === 'function')
       onstep = opts.step;
     //preserve = (opts.preserve ? true : false);
@@ -1056,7 +1061,12 @@ function fastXfer(src, dst, srcPath, dstPath, opts, cb) {
 
     srcHandle = sourceHandle;
 
-    src.fstat(srcHandle, function tryStat(err, attrs) {
+    if (fileSize !== undefined)
+      src.fstat(srcHandle, tryStat);
+    else
+      tryStat(null, { size: fileSize });
+
+    function tryStat(err, attrs) {
       if (err) {
         if (src !== fs) {
           // Try stat() for sftp servers that may not support fstat() for
@@ -1192,7 +1202,7 @@ function fastXfer(src, dst, srcPath, dstPath, opts, cb) {
           }
         }
       });
-    });
+    }
   });
 }
 SFTPStream.prototype.fastGet = function(remotePath, localPath, opts, cb) {