[MERGE] fix blackbox failures when run from windows executable

[Andrew Bennetts]

Andrew Bennetts wrote:
[...]
> Hmm, I haven't tried running your last patch, but this one fails for me.  So

It happens with your first patch too.  Either way I get a failure like:

Traceback (most recent call last):
  File "/home/andrew/warthogs/bzr/mhammond/bzrlib/tests/test_plugins.py", line 474, in test_set_plugins_path_with_trailing_slashes
    bzrlib.plugins.__path__[:len(expected_path)])
AssertionError: not equal:
a = ['first', 'second', '/usr/lib/python2.5/site-packages/bzrlib/plugins']
b = ['first', 'second', '/home/andrew/warthogs/bzr/mhammond/bzrlib/plugins']

Maybe this due to running the test suite out of a checkout while also having
bzrlib installed system wide?

-Andrew.