unpack-trees: refuse to remove startup_info->original_cwd

unpack-trees: refuse to remove startup_info->original_cwd

In the past, when a directory needs to be removed to make room for a
file, we have always errored out when that directory contains any
untracked (but not ignored) files.  Add an extra condition on that: also
error out if the directory is the current working directory we inherited
from our parent process.

Acked-by: Derrick Stolee <redacted>
Acked-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>