As reported on github #4, it is possible that with mixed-VCS repos
sunrise-commit will prefer a different VCS than echangelog would do. To
avoid that, always pass our VCS to echangelog.
Now ChangeLog creation is controlled directly by a dedicated variable.
It is disabled by default, and enabled implicitly with cvs, svn and
git-svn. It can be enabled explicitly via `-c' too.
SC_VCS no longer will contain git-svn -- it will be simply git. That
simplifies a lot of checks in the code.
