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

This patch removes a chunk of code (the Git::SVN::Fetcher consumer of
libsvn's tree delta protocol) from git-svn.perl and documents its
interface so the hurried reader does not have to read that code right
away.

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

Author: jrn