This code essentially replaces part of ref with another ref, for example
'@{-1}@{u}' is replaced with 'master@{u}', but this can be reused for
other purposes other than nth prior checkouts.
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>