stash: do not attempt to remove startup_info->original_cwd
Since stash spawns a `clean` subprocess, make sure we run that from the
startup_info->original_cwd directory, so that the `clean` processs knows
to protect that directory. Also, since the `clean` command might no
longer run from the toplevel, pass the ':/' magic pathspec to ensure we
still clean from the toplevel.
Acked-by: Derrick Stolee <stolee@gmail.com> Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>