read-cache.c: move tempfile creation/cleanup out of write_shared_index

read-cache.c: move tempfile creation/cleanup out of write_shared_index

For one thing, we have more consistent cleanup procedure now and always
keep errno intact.

The real purpose is the ability to break out of write_locked_index()
early when mks_tempfile() fails in the next patch. It's more awkward to
do it if this mks_tempfile() is still inside write_shared_index().

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>