[BUG][Fix] (win32?) merge raise error when path-to-branch is relative

[Alexander Belchenko]

Bug reproduction:

$ bzr init a
$ bzr init b
$ cd b
$ bzr merge ../a
bzr: ERROR: exceptions.IOError: [Errno 13] Permission denied: u'../a'
   at D:\user\python\bzr\devel\bzr.dev\bzrlib\builtins.py line 2057
   in run


line where exception raised is:

             if branch is not None:
                 reader = BundleReader(file(branch, 'rb'))    # <<< 2057
             else:
                 reader = None

My first thoughts to fix this was:

             if branch is not None and not os.path.isdir(branch):
                 reader = BundleReader(file(branch, 'rb'))
             else:
                 reader = None

This change is trivial and boring to me to write appropriate test.

--
Alexander