git-svn: rename SVN::Git::* packages to Git::SVN::*

git-svn: rename SVN::Git::* packages to Git::SVN::*

Using names in the Git:: namespace means these cannot conflict with a
hypothetical binding teaching Subversion to interact with git
repositories.

Currently the packages are private to git-svn.perl so the choice of
name isn't likely to make much difference.  This change is mainly
meant as preparation for splitting out the packages in question as
modules on the public search path.

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