We used to do this so could pass a mutable string to
enter_repo. But since 1c64b48 (enter_repo: do not modify
input, 2011-10-04), this is not necessary.
The resulting code is simpler, and it fixes a minor leak.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>