src/pull: Support local pulls for collection–refs

src/pull: Support local pulls for collection–refs

Previously, collection–refs could only be pulled from a repository if it
had a summary file (which listed them). There was no way to pull from a
local repository which doesn’t have a summary file, and where the refs
were stored as refs/remotes/$remote/$ref, with a config section linking
that $remote to the queried collection ID.

Fix that by explicitly supporting pull_data->remote_repo_local in
fetch_ref_contents().

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #1182
Approved by: cgwalters