[MERGE][0.14] Allow 'bzr push' to push onto existing dir

[Robert Collins]

Robert Collins has voted +1 (conditional).
Status is now: Conditionally approved
Comment:
I think perhaps that alexanders error indicates a bug though - it may be 
that --use-existing prevents the code path for reuse of a bzrdir control 
dir working correctly - or may be in the wrong order.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C45A3F5DA.6040007%40arbash-meinel.com%3E