commit: discard index after setting up partial commit

commit: discard index after setting up partial commit

There may still be some entries from the original index that
should be discarded before we show the status. In
particular, if a file was added in the index but not
included in the partial commit, it would still show up in
the status listing as staged for commit.

Ultimately the correct fix is to keep the two states in
separate index_state variables. Then we can avoid having
to reload the cache from the temporary file altogether, and
just point wt_status_print at the correct index.

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