Traceback on `git bzr push --overwrite`

[OddBloke]

$ git bzr push --overwrite
09:17:16 Starting import ...
09:17:17 Updating branch information ...
     branch cpc now has 1062 revisions and 0 tags
09:17:17 Imported 32 revisions, updating 1 branch and 0 trees in 0:00:01
To refresh the working tree for other branches, use 'bzr update' inside that branch.
Traceback (most recent call last):
  File "/usr/bin/git-bzr", line 709, in <module>
    sys.exit(main(sys.argv))
  File "/usr/bin/git-bzr", line 703, in main
    return func(argv[2:])
  File "/usr/bin/git-bzr", line 486, in cmd_push
    bzr(['push'] + params + [upstream])
  File "/usr/bin/git-bzr", line 63, in bzr
    return run_command(cmd, **kwargs)
  File "/usr/bin/git-bzr", line 33, in run_command
    logging.info(' '.join(cmd))
TypeError: sequence item 3: expected string, NoneType found

[OddBloke]

It looks like this is a problem in cmd_push(), as upstream is never set to anything but None before passing it to bzr(); it is assumed that having a known_upstream means there won't be errors, but giving --overwrite bypasses line 459 which actually does upstream = known_upstream.

This bug is either with reporting the error to the user (i.e. git-bzr-ng should be saying "You have to specify which branch you want to overwrite.") or with the actual behaviour (i.e. git-bzr-ng should be overwriting the known upstream branch).

@termie, I was expecting (and would, I think, prefer) the latter, but would be happy to submit a PR for whichever you think makes sense. Let me know. :)