avoid padding content in serveGet

Always truncate instead. The padding risked something not noticing the
content was bad and getting a file that was corrupted in a novel way
with the padding "X" at the end. A truncated file is better.

doc/todo/git-annex_proxies.mdwn

@@ -28,6 +28,10 @@ Planned schedule of work:
 
 ## work notes
 
+* servePut and clientPut pad the data to indicate when it's not valid.
+  That should not be necessary, they should always be able to truncate the
+  data. 
+
 * Implement: servePutOffset, serveLockContent
 
 * A Locker should expire the lock on its own after 10 minutes initially.