[MERGE] remerge command works on win32

[Aaron Bentley]

Martin Pool wrote:

> The original code here has a bug - it will absorb a KeyboardInterrupt or
> similar error that happens to arrive during finalization.  Let's try to
> fix it rather than just moving it around.
> 
> Probably the tree finalization can be inside a nested try/finally block,
> so that the lock cleanup will be done even if an error happens in
> finalization?

I think my original sentiment was that it would suck to error out 
because we couldn't clean up the limbo directory, even though the merge 
had succeeded.

I can change this, sure.  We may get more errors on Win32, but those 
errors would have manifested on the next merge/revert command anyhow.

Aaron