lib: Add an API to list only "our" objects, fix prune to use it

When doing a prune, we should not try to delete objects in parent
repos, since it'll fail.  There is a bigger discussion about the
semantics of `parent=` to be had, but this will fix trying to use
`ostree prune --repo=/ostree/repo/extensions/rpmostree/pkgcache`.

Closes: https://github.com/ostreedev/ostree/issues/467

Closes: #471
Approved by: jlebon

src/libostree/ostree-repo-prune.c

@@ -337,8 +337,8 @@ ostree_repo_prune (OstreeRepo        *self,
         }
     }
 
-  if (!ostree_repo_list_objects (self, OSTREE_REPO_LIST_OBJECTS_ALL, &objects,
-                                 cancellable, error))
+  if (!ostree_repo_list_objects (self, OSTREE_REPO_LIST_OBJECTS_ALL | OSTREE_REPO_LIST_OBJECTS_NO_PARENTS,
+                                 &objects, cancellable, error))
     goto out;
 
   if (!refs_only)

src/libostree/ostree-repo.c

@@ -3590,7 +3590,7 @@ ostree_repo_list_objects (OstreeRepo                  *self,
     {
       if (!list_loose_objects (self, ret_objects, NULL, cancellable, error))
         goto out;
-      if (self->parent_repo)
+      if ((flags & OSTREE_REPO_LIST_OBJECTS_NO_PARENTS) == 0 && self->parent_repo)
         {
           if (!list_loose_objects (self->parent_repo, ret_objects, NULL, cancellable, error))
             goto out;

src/libostree/ostree-repo.h

@@ -798,11 +798,13 @@ gboolean       ostree_repo_read_commit (OstreeRepo    *self,
  * @OSTREE_REPO_LIST_OBJECTS_LOOSE: List only loose (plain file) objects
  * @OSTREE_REPO_LIST_OBJECTS_PACKED: List only packed (compacted into blobs) objects
  * @OSTREE_REPO_LIST_OBJECTS_ALL: List all objects
+ * @OSTREE_REPO_LIST_OBJECTS_NO_PARENTS: Only list objects in this repo, not parents
  */
 typedef enum {
   OSTREE_REPO_LIST_OBJECTS_LOOSE = (1 << 0),
   OSTREE_REPO_LIST_OBJECTS_PACKED = (1 << 1),
-  OSTREE_REPO_LIST_OBJECTS_ALL = (1 << 2)
+  OSTREE_REPO_LIST_OBJECTS_ALL = (1 << 2),
+  OSTREE_REPO_LIST_OBJECTS_NO_PARENTS = (1 << 3),
 } OstreeRepoListObjectsFlags;
 
 /**