clone: support 'clone --shared' from a worktree

clone: support 'clone --shared' from a worktree

When worktree functionality was originally implemented, the possibility
of 'clone --local' from within a worktree was overlooked, with the
result that the location of the "objects" directory of the source
repository was computed incorrectly, thus the objects could not be
copied or hard-linked by the clone. This shortcoming was addressed by
744e469755 (clone: allow --local from a linked checkout, 2015-09-28).

However, the related case of 'clone --shared' (despite being handled
only a few lines away from the 'clone --local' case) was not fixed by
744e469755, with a similar result of the "objects" directory location
being incorrectly computed for insertion into the 'alternates' file.
Fix this.

Reported-by: Marc-André Lureau <marcandre.lureau@gmail.com>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>