Improve `guix pull --commit=` behaviour when with channels

[pkill9]

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?