[MERGE] Teach transport.put_file to return the size written

[Martin Pool]

Martin Pool has voted approve.
Status is now: Approved
Comment:
That seems reasonable.

Incidentally, maybe we should consider it evil or deprecated to pass a 
stringio into put_file, rather than calling put_bytes?  There may be 
some old code that needlessly constructs the wrapper.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C1190664929.8901.1.camel%40lifeless-64%3E