add stash completions to git show and git diff

[m4rch3n1ng]

Description

you can use git show and git diff with stashes

$ git show stash@{1}
$ git diff stash@{0} stash@{1}
$ git diff stash@{0}

this adds completions to those
especially the former (git show) is fairly useful because git stash show only shows like with --stat

notes:

• you can also git checkout and git switch --detach (to) stashes, but i didn't want to encourage that, as you should probably use git stash apply or git stash pop, so you aren't missing new commits you added since creating the stash. if you want me to i can add that as well
• technically this should also apply to git cherry-pick (and probably git cherry) but that should probably be done via git stash apply, so i didn't add it

$ git cherry-pick stash@{1} -m 1

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[zanchey]

Thanks!