stash: allow stashes to be referenced by index only

stash: allow stashes to be referenced by index only

Instead of referencing "stash@{n}" explicitly, make it possible to
simply reference as "n".  Most users only reference stashes by their
position in the stash stack (what I refer to as the "index" here).

The syntax for the typical stash (stash@{n}) is slightly annoying and
easy to forget, and sometimes difficult to escape properly in a
script. Because of this the capability to do things with the stash by
simply referencing the index is desirable.

This patch includes the superior implementation provided by Øsse Walle
(thanks for that), with a slight change to fix a broken test in the test
suite. I also merged the test scripts as suggested by Jeff King, and
un-wrapped the documentation as suggested by Junio Hamano.

Signed-off-by: Aaron M Watson <watsona4@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>