submodule update --init: display correct path from submodule

submodule update --init: display correct path from submodule

In the submodule helper we did not correctly handled the display path
for initializing submodules when both the submodule is inside a
subdirectory as well as the command being invoked from a subdirectory
(as viewed from the superproject).

This was broken in 3604242f080, which was written at a time where
there was no super-prefix available, so we abused the --prefix option
for the same purpose and could get only one case right (the call from
within a subdirectory, not the submodule being in a subdirectory).

Test-provided-by: David Turner <novalis@novalis.org>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>