bzr-buildpackage fails if directory contains umlauts

Running bzr-buildpackage in an UTF environment with umlauts in directory path causes an error in bzr-buildpackage.

Easily reproducible by running the following:
===================

/Arbeitsfläche/bugs/indicator-messages$ bzr bd -- -S -us -uc

Produces:
==========

Building using working tree
Building package in normal mode
Looking for a way to retrieve the upstream tarball
Using pristine-tar to reconstruct indicator-messages_12.10.6daily13.06.19.orig.tar.gz.
bzr: ERROR: exceptions.UnicodeEncodeError: 'ascii' codec can't encode character u'\xe4' in position 40: ordinal not in range(128)

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 930, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 1121, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 673, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 697, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/cleanup.py", line 136, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/cleanup.py", line 166, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/builddeb/cmds.py", line 449, in run
    builder.export()
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/builddeb/builder.py", line 77, in export
    self.distiller.distill(self.target_dir)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/builddeb/source_distiller.py", line 79, in distill
    self._distill(target)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/builddeb/source_distiller.py", line 107, in _distill
    export(self.tree, target)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/builddeb/util.py", line 603, in export
    filtered=filtered, per_file_timestamps=True)
  File "/usr/lib/python2.7/dist-packages/bzrlib/export/__init__.py", line 175, in export
    per_file_timestamps, fileobj):
  File "/usr/lib/python2.7/dist-packages/bzrlib/export/__init__.py", line 128, in get_export_generator
    trace.mutter('export version %r', tree)
  File "/usr/lib/python2.7/dist-packages/bzrlib/trace.py", line 155, in mutter
    out = fmt % tuple(real_args)
UnicodeEncodeError: 'ascii' codec can't encode character u'\xe4' in position 40: ordinal not in range(128)

bzr 2.6.0dev3 on python 2.7.4 (Linux-3.8.0-25-generic-x86_64-with-
    Ubuntu-13.04-raring)
arguments: ['/usr/bin/bzr', 'bd', '--', '-S', '-us', '-uc']
plugins: bash_completion[2.6.0dev3], builddeb[2.7.0dev],
    changelog_merge[2.6.0dev3], grep[2.6.0dev3], launchpad[2.6.0dev3],
    netrc_credential_store[2.6.0dev3], news_merge[2.6.0dev3],
    po_merge[2.6.0dev3], weave_fmt[2.6.0dev3]
encoding: 'utf-8', fsenc: 'UTF-8', lang: 'de_DE.UTF-8'

*** Bazaar has encountered an internal error. This probably indicates a
    bug in Bazaar. You can help us fix it by filing a bug report at
        https://bugs.launchpad.net/bzr/+filebug
    including this traceback and a description of the problem.

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: bzr 2.6.0~bzr6571-4ubuntu2
ProcVersionSignature: Ubuntu 3.8.0-25.37-generic 3.8.13
Uname: Linux 3.8.0-25-generic x86_64
ApportVersion: 2.9.2-0ubuntu8.1
Architecture: amd64
Date: Wed Jun 26 22:13:56 2013
EcryptfsInUse: Yes
InstallationDate: Installed on 2013-06-21 (5 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: bzr
UpgradeStatus: No upgrade log present (probably fresh install)