Fixes for push corner case
TAH found a bug where push would push things the remote already had,
raising an assertion. This turned out to be because the changeset
protocol was not recording a common subset node in one case.
Also discovered was that the protocol was queueing multiple copies of
a node for pull. Fixed by changing fetch to a hash.
Add some more debugging output.
abort: repository a/.hg not found!
255
requesting all changes
abort: error: Connection refused
255
abort: repository a/.hg not found!
255
abort: destination '../a' already exists
1
abort: repository a/.hg not found!
255
abort: destination 'q' already exists