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

src/libostree/ostree-repo-private.h

@@ -265,6 +265,7 @@ _ostree_repo_write_ref (OstreeRepo                 *self,
                         const char                 *remote,
                         const OstreeCollectionRef  *ref,
                         const char                 *rev,
+                        const char                 *alias,
                         GCancellable               *cancellable,
                         GError                    **error);