[feature request] stash diff usability

I can't recall wanting to get, from the command line, the contents of a stash as a diff in the past. I thought there was maybe an option to `git stash show` , boy was I wrong: `git diff 'stash@{0}^' 'stash@{0}'`. This seems rather user hostile.

I see the request to get the "!" syntax working, this would be an improvement, However, I'd like to see the command `git stash diff [stashname]` to be a shorthand for the above, and of course defaulting to the most recent stash.

My apologies if this matches an old, previously rejected suggestion. Searching the mailing list exhaustively takes some skill I haven't acquired.