When extra channels have been added, running `guix pull --commit=<commit>` returns "guix pull: error: '--url', '--commit', and '--branch' are not applicable". I think it would be better if these flags only affected the "guix" channel, including when extra channels have been added.
This makes sense considering `guix pull` fails when no channel with the name "guix" exists - if you provide a list of channels in channels.scm and none of them are named "guix", `guix pull` will fail (a channel named "guix" is provided in %default-channels). Thoughts?
