[RFC] best way to show diff of commit

  Hi there,

  There is specific script I run in my vim with git, that tries to show
from the 'status' git commit shows in the buffer which list of files has
changed, and builds a diff from it quite clumsily[0].

  I wonder how hard it would be for git commit to "stash" the current
commit being sent somewhere editors would be able to have a look at (an
alternate index is probably fine). Note that maybe I'm stupid and
overlooked that such a thing already exists.  I'd like to have it in two
flavors: normal and amend mode. normal mode would show what the
resulting commit diff looks like, and the amend mode only shows the
incrementall diff the amend adds to the previous commit.

  My question is: what do you think is the best way to do that, and
where ?

  [0] the issue with this approach is that it's completely broken in
      amending mode (does not shows the proper thing), and the generated
      diffs aren't excellent, because as an editor plugin, it's hard to
      treat renames and copies easily, so I generate really really nasty
      diffs in that case too.