git-svn: make Git::SVN::Editor a separate file

This makes the git-svn script shorter and less scary for beginners to
read through for the first time.  Take the opportunity to explain the
purpose and basic interface of the Git::SVN::Editor class while at it.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Eric Wong <normalperson@yhbt.net>

Author: jrn