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

This slices off another 600 or so lines from the frighteningly long
git-svn.perl script.

The Git::SVN::Ra interface is similar enough to SVN::Ra that it is
probably safe to ignore most of its implementation on first reading.
(Documenting or moving functions that do not fit that pattern is left
as an exercise to the interested reader.)

[ew: rebased and fixed conflict against
 commit c26ddce
 (git-svn: platform auth providers are working only on 1.6.15 or newer)]

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

Author: jrn