When running Bazaar or Breezy, we construct a plugin path in order to

register rbssh as an SSH provider. To avoid overriding existing

plugins, we factor in an existing path in the environment. If that's

empty, the plugin path ends up with a trailing :.
Breezy seems to, intentionally or otherwise, treat an empty path after

the : as "current directory." This can cause failures if trying to run

from within a Python module path (which can happen in unit test runs).
We now strip away any trailing :, from either our provided path or an

existing plugin path, in order to avoid this. We also no longer convert

to UTF-8, as SCMTool.popen() normalizes environment variables for us.
While working on this, I found that we sometimes also joined the

repository path to itself when trying to validate a repository. This

actually worked, which is why we didn't notice it before (presumably,

Bazaar and Breezy handle the path until it finds a Bazaar repository,

ignoring the rest). However, it's clearly wrong. We no longer perform

this join for repository validation.

Unit tests now pass. This was discovered on my release-5.0.x tree, but

is being backported to release-4.0.x.