Converting to subversion repository is not preserving original commit dates as
it may break some subversion functionality if commit dates are not monotonically
increasing.

This patch adds `convert.svn.dangerous-set-commit-dates` configuration option
to change this behaviour and enable commit dates convertion for those who want
to take risks.

Subversion always uses commit dates with UTC timezone, so only timestamps
are used.

Test `test-convert-svn-sink.t` uses `svnxml.py` script to dump history of svn
repositories. Atm the script is not printing `date` field from svn log. This
patch changes this to allow checks on correctness of date and time convertion.

Documentation is updated. Additional test case is added to test commit dates
convertion.

Differential Revision: https://phab.mercurial-scm.org/D9721