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.h

@@ -327,6 +327,14 @@ gboolean      ostree_repo_set_ref_immediate (OstreeRepo *self,
                                              GCancellable  *cancellable,
                                              GError       **error);
 
+_OSTREE_PUBLIC
+gboolean      ostree_repo_set_alias_ref_immediate (OstreeRepo *self,
+                                                   const char *remote,
+                                                   const char *ref,
+                                                   const char *target,
+                                                   GCancellable  *cancellable,
+                                                   GError       **error);
+
 #ifdef OSTREE_ENABLE_EXPERIMENTAL_API
 
 _OSTREE_PUBLIC
@@ -452,9 +460,11 @@ gboolean      ostree_repo_list_refs (OstreeRepo       *self,
 /**
  * OstreeRepoListRefsExtFlags:
  * @OSTREE_REPO_LIST_REFS_EXT_NONE: No flags.
+ * @OSTREE_REPO_LIST_REFS_EXT_ALIASES: Only list aliases.  Since: 2017.10
  */
 typedef enum {
   OSTREE_REPO_LIST_REFS_EXT_NONE = 0,
+  OSTREE_REPO_LIST_REFS_EXT_ALIASES = 1,
 } OstreeRepoListRefsExtFlags;
 
 _OSTREE_PUBLIC