repo-info file has wrong value for the branch field

[Felipe Reyes]

The repo-info file is dropped into the charm's archive before pushing into the charm store, we use it a lot to double check if an specific commit id is part of that version.

But the branch field says "HEAD" when I would expect (at the moment of writing this) it says "stable/18.02" and "master" for charms pulled from the ~openstack-charmers-next namespace

$ curl -s https://api.jujucharms.com/charmstore/v5/heat/archive/repo-info
commit-sha-1: c3fcab00c93f9f180b0859a590e9d73f82bf66e0
commit-short: c3fcab0
branch: HEAD
remote: https://github.com/openstack/charm-heat
info-generated: Tue May 8 14:39:23 UTC 2018
note: This file should exist only in a built or released charm artifact (not in the charm source code tree).

$ curl -s https://api.jujucharms.com/charmstore/v5/~openstack-charmers-next/heat/archive/repo-info
commit-sha-1: 5f8d3317e6dd9b67861bab10258f0290281339fc
commit-short: 5f8d331
branch: HEAD
remote: https://github.com/openstack/charm-heat
info-generated: Thu Apr 19 15:51:40 UTC 2018
note: This file should exist only in a built or released charm artifact (not in the charm source code tree).