lib: Add a public helper method for pruning to find all ref'd commits

Prep for reworking how we do sysroot cleanup.  We're going to
start doing more lowlevel pruning work there, and I wanted to avoid
duplicating the ref enumeration.

Closes: #1566
Approved by: jlebon

src/libostree/ostree-repo.h

@@ -1202,6 +1202,14 @@ struct _OstreeRepoPruneOptions {
 
 typedef struct _OstreeRepoPruneOptions OstreeRepoPruneOptions;
 
+_OSTREE_PUBLIC
+gboolean
+ostree_repo_traverse_reachable_refs (OstreeRepo *self,
+                                     guint       depth,
+                                     GHashTable *reachable,
+                                     GCancellable *cancellable,
+                                     GError      **error);
+
 _OSTREE_PUBLIC
 gboolean ostree_repo_prune_from_reachable (OstreeRepo             *self,
                                            OstreeRepoPruneOptions *options,