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 <[email protected]>
Signed-off-by: Eric Wong <[email protected]>

Author: jrn