Feature: Staging from a stash

I discovered an unexpected behaviour, if you show a stash you can stage (and apparently unstage, according to ?) changes from the stash view. Presumably ? should show what is actually possible, since unstaging changes from the stash doesn't make much sense.

Staging a file tries to do git add the_file which usually doesn't do anything.

Staging a hunk will apply a reverse patch of the hunk in the stash. Which is quite weird because the diff is "changing" something that never existed in the first place, if you then unstage that then the change disappears altogether.

When I first did this, I thought I'd discovered an amazing feature because I often have stashes that are not well-factored, and being able to stage individual pieces out of them without having to unstash (and deal with potential conflicts I dont' care about) would be incredible.

Start at the stash view and inspect the staging and unstaging actions, including the ? help output. Trace how staging a file and hunk is handled, then define the supported behavior for stash changes and update the help text so the completed behavior matches it.