Prepare Git::SVN for extraction into its own file.

Prepare Git::SVN for extraction into its own file.

This means it should be able to load without git-svn being loaded.

* Load Git.pm on its own and all the needed command functions.

* It needs to grab at a git-svn lexical $_prefix representing the --prefix
  option.  Provide opt_prefix() for that.  This is a refactoring artifact.
  The prefix should really be passed into Git::SVN->new.

* Unqualify unnecessarily fully qualified globals like
  $Git::SVN::default_repo_id.

* Lexically isolate the class just to make sure nothing is leaking out.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Eric Wong <normalperson@yhbt.net>