bug#32618: Channels: guix pull error when specifying branch

[Eric Brown]

I am trying the new Channels, working with a
~/.config/guix/channels.scm:

`guix pull' completes when a commit is specified:
(list (channel
       (name 'guix)
       (url "https://git.savannah.gnu.org/git/guix.git";)
       (commit "391097782311c82c04b171db7373a0bbfc1d8653")))

but not when the branch name is specified:
(list (channel
       (name 'guix)
       (url "https://git.savannah.gnu.org/git/guix.git";)
       (branch "master")))

guix pull: error: Git error: cannot locate remote-tracking branch 'master'

I have tested with my own site as well:
       (url "https://ericcbrown.com:8888/ecbrown/guix.git";)

and I see the same behavior.