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

tests/pull-test.sh

@@ -508,5 +508,5 @@ rm ostree-srv/gnomerepo/summary
 if ${CMD_PREFIX} ostree --repo=repo pull origin main 2>err.txt; then
     fatal "pull of invalid ref succeeded"
 fi
-assert_file_has_content_literal err.txt 'error: Fetching refs/heads/main: Invalid rev lots of html here  lots of html here  lots of html here  lots of'
+assert_file_has_content_literal err.txt 'error: Fetching checksum for ref ((empty), main): Invalid rev lots of html here  lots of html here  lots of html here  lots of'
 echo "ok pull got HTML for a ref"