enter_repo: do not modify input

enter_repo: do not modify input

entr_repo(..., 0) currently modifies the input to strip away
trailing slashes. This means that we some times need to copy the
input to keep the original.

Change it to unconditionally copy it into the used_path buffer so
we can safely use the input without having to copy it. Also store
a working copy in validated_path up-front before we start
resolving anything.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Phil Hord <hordp@cisco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>