get_sha1: teach ":$n:<path>" the same relative path logic

get_sha1: teach ":$n:<path>" the same relative path logic

We taught the object name parser to take ":./<path>", ":../<path>", etc.
and understand them to be relative to the current working directory.

Given that ":<path>" is just a short-hand for ":0:<path>" (i.e. "take
stage #0 of that path"), we should allow ":$n:<path>" to interpret them
the same way.

Signed-off-by: Junio C Hamano <gitster@pobox.com>