Use a temporary index for git commit --interactive

Use a temporary index for git commit --interactive

Change the behaviour of git commit --interactive so that when you abort
the commit (by leaving the commit message empty) the index remains
unchanged.

Hitherto an aborted commit --interactive has added the selected hunks to
the index regardless of whether the commit succeeded or not.

Signed-off-by: Conrad Irwin <conrad.irwin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>