gexport fails on bookmarks with spaces in them (RefFormatError)

Created originally on Bitbucket by Zoltán Lehóczky

If a bookmark in the hg repo contains a space gexport will fail. E.g. the bookmark "hg/Orchard1.9.1" will be handled fine (and appear as a git branch), the bookmark "hg/Orchard 1.9.1" will fail with the below exception.

** Unknown exception encountered with possibly-broken third-party extension hggit
** which supports versions 3.4 of Mercurial.
** Please disable hggit and try your action again.
** If that fixes the bug please report it to https://bitbucket.org/durin42/hg-git/issues
** Python 2.7.9 (default, Dec 10 2014, 12:28:03) [MSC v.1500 64 bit (AMD64)]
** Mercurial Distributed SCM (version 3.5+5)
** Extensions loaded: rebase, strip, mq, hggit, eol
Traceback (most recent call last):
  File "hg", line 49, in <module>
  File "mercurial\dispatch.pyo", line 30, in run
  File "mercurial\dispatch.pyo", line 92, in dispatch
  File "mercurial\dispatch.pyo", line 163, in _runcatch
  File "mercurial\dispatch.pyo", line 895, in _dispatch
  File "mercurial\dispatch.pyo", line 656, in runcommand
  File "mercurial\dispatch.pyo", line 1013, in _runcommand
  File "mercurial\dispatch.pyo", line 982, in checkargs
  File "mercurial\dispatch.pyo", line 892, in <lambda>
  File "mercurial\util.pyo", line 792, in check
  File "mercurial\extensions.pyo", line 171, in closure
  File "mercurial\util.pyo", line 792, in check
  File "hgext\mq.pyo", line 3515, in mqcommand
  File "mercurial\util.pyo", line 792, in check
  File "hggit\__init__.pyo", line 182, in gexport
  File "hggit\git_handler.pyo", line 294, in export_commits
  File "hggit\git_handler.pyo", line 1187, in update_references
  File "dulwich\refs.pyo", line 261, in __setitem__
  File "dulwich\refs.pyo", line 565, in set_if_equals
  File "dulwich\refs.pyo", line 174, in _check_refname
dulwich.errors.RefFormatError: refs/heads/hg/Orchard 1.9.1

This is possibly the same bug as this, but not sure: https://bitbucket.org/durin42/hg-git/issues/116/exception-pushing-to-git-repository

I realize this is more of a dulwich issue but not entirely sure about it, so posting it here.