detached-stash: simplify git stash show

detached-stash: simplify git stash show

This commit refactors git stash show to make use of the assert_stash_like function.

git show now dies if the presented argument is non-stash-like.

Previous behaviour was to tolerate commits that were not even stash-like.

Previously, git stash show would accept stash-like arguments, but
only if there was a stash on the stack.

Now, git stash accepts stash-like arguments always and only fails
if no stash-like argument is specified and there is no stash stack.

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