Commit of currently installed package?

[t@fullmeta.me]

Hi guix.

Is there a way to find which commit the installed package came 
from. So that I could, for instance, `guix shell` this exact 
package. Specific case where I'm at a loss is this.

At one point I installed emacs-next --with-branch=master. Well, 
I've no clue what that master was pointing at at the time. I find 
myself in the situation where current upstream master appears 
subtly broken, but my local install is fine. But how do I get my 
currently installed emacs-next version run inside `guix shell` or 
`guix shell --container`?

--export-manifest is approximate and doesn't even give package 
  versions. Even if I start playing games with --export-channels 
  or Scheme %current-profile => manifest => lookup in inferior 
  that would still find whatever guix repo commit had for 
  emacs-next, not what I actually have installed.

Does guix track that sort of provenance for me to extract?

Thanks