[MERGE] fix blackbox failures when run from windows executable

[Andrew Bennetts]

Mark Hammond wrote:
[...]
> > This change looks a bit weird, particularly how you set
> > bzrlib.plugins.__path__
> > to [] multiple times.  I think it's correct, but confusing.  I think if
> > you extract the first determine the default builtin path section into a
> > helper method it would be a little clearer.
> 
> Thinking more about it, that code was simply trying to get the builtin plugin
> path, which is always at the end of bzrlib.__path__ - so I simplified the test
> and added more comments about this.

Hmm, I haven't tried running your last patch, but this one fails for me.  So
unfortunately:

bb:resubmit

Interestingly, I get a slightly different failure if I run "bzr selftest -s
bzrlib.tests.test_plugins" vs. "bzr selftest -s bzrlib.tests.test_plugins
test_set_plugins_path_with_trailing_slashes".

-Andrew.