convert/svn: abort operation when python bindings are not available
Subversion python bindings check was not present in svn_sink source
class which made it fail while using svn as destination repository.
Added a more maintainble svn bindings check for svn_source and svn_sink
classes.
.. 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