According to the comment, enter_repo will modify its input.
However, this has not been the case since 1c64b48
(enter_repo: do not modify input, 2011-10-04). Drop the
now-useless copy.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>