for_each_alternate_ref: stop trimming trailing slashes

for_each_alternate_ref: stop trimming trailing slashes

The real_pathdup() function will have removed extra slashes
for us already (on top of the normalize_path() done when we
created the alternate_object_database struct in the first
place).

Incidentally, this also fixes the case where the path is
just "/", which would read off the start of the array.
That doesn't seem possible to trigger in practice, though,
as link_alt_odb_entry() blindly eats trailing slashes,
including a bare "/".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>