bookmarks: always write undo file
Always write the undo file. Otherwise, rollback will not work for
the initial bookmark as undo.bookmarks doesn't exists. In this case
undo.bookmarks needs already be empty.
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html