detached-stash: tests of git stash with stash-like arguments

detached-stash: tests of git stash with stash-like arguments

Adds new tests which check that:
* git stash branch handles a stash-like argument when there is a stash stack
* git stash branch handles a stash-like argument when there is not a stash stack
* git stash show handles a stash-like argument when there is a stash stack
* git stash show handles a stash-like argument when there is not a stash stack
* git stash drop fails early if the specified argument is not a stash reference
* git stash pop fails early if the specified argument is not a stash reference
* git stash * fails early if the reference supplied is bogus
* git stash fails early with stash@{n} where n >= length of stash log

Helped-by: Johannes Sixt
Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>