[MERGE] multiparent.py: workaround for windows bug: .tell() for files opened in 'ab' mode before any write returns 0

[Aaron Bentley]

Aaron Bentley has voted approve.
Status is now: Approved
Comment:
This looks good.  Now that I see this, I remember we do this in other 
places too.  It makes me wonder whether we can factor this out, but 
without knowing how a file's going to be used, it's pretty tricky.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C46F2E830.9060200%40ukr.net%3E