apply/reverse changes with `a`/`v`

[altsem]

Currently it's possible to apply / reverse changes while looking at a commit using s (stage) and u (unstage). This has some undesired effects though, as they leave unstaged and staged changes.

Implement a and v as in Magit. ~Disable s and u while looking at logs/commits for now.~ (Magit lets you do this.)