git-svn: convert to using Git.pm

Thanks to Git.pm, I've been able to greatly reduce the amount
of extra work that needs to be done to manage input/output
pipes in Perl.

chomp usage has also been greatly reduced, too.

All tests (including full-svn-test) still pass, but this has
not been tested extensively in the real-world.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>