lib/repo: Add API to create and list ref aliases

There are multiple use cases where we'd like to alias refs.

First, having a "stable" alias which gets swapped across major
versions: https://pagure.io/atomic-wg/issue/228

Another case is when a ref is obsoleted;
<https://pagure.io/atomic-wg/issue/303>
This second one could be done with endoflife rebase, but I think
this case is better on the server side, as we might later change
our minds and do actual releases there.

I initially just added some test cases for symlinks in the `refs/heads` dir to
ensure this actually works (and it did), but I think it's worth having APIs.

Closes: #1033
Approved by: jlebon

apidoc/ostree-sections.txt

@@ -307,6 +307,7 @@ ostree_repo_abort_transaction
 ostree_repo_transaction_set_refspec
 ostree_repo_transaction_set_ref
 ostree_repo_set_ref_immediate
+ostree_repo_set_alias_ref_immediate
 ostree_repo_set_cache_dir
 ostree_repo_sign_delta
 ostree_repo_has_object