Allow helper to map private ref names into normal names
This allows a helper to say that, when it handles "import
refs/heads/topic", the script it outputs will actually write to
refs/svn/origin/branches/topic; therefore, transport-helper should
read it from the latter location after git-fast-import completes.
Signed-off-by: Daniel Barkalow <barkalow@iabervon.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>