In magit-wip-save, expand the index file name

Git is creating the index in the git topdir, but magit-wip-save was
deleting it in the current directory, and so failed. By expanding the
file-name of the temporary index, we protect ourself of this.
Merge pull request #1359 from vanicat/t/magit-wip-bug-fix

magit-wip-save is failing with no such file of directory /something/index4176xhf