pull: Support --mirror option

There's several use cases for calling into ostree itself to do
mirroring, instead of using bare rsync.  For example, it's a bit more
efficient as it doesn't require syncing the objects/ directory.

https://bugzilla.gnome.org/show_bug.cgi?id=728351

src/libostree/ostree-repo-pull.c

@@ -1262,8 +1262,8 @@ ostree_repo_pull (OstreeRepo               *self,
         }
       else
         {
-          ostree_repo_transaction_set_ref (pull_data->repo, pull_data->remote_name, ref, checksum);
-
+          gboolean is_mirror = (pull_data->flags & OSTREE_REPO_PULL_FLAGS_MIRROR) > 0;
+          ostree_repo_transaction_set_ref (pull_data->repo, is_mirror ? NULL : pull_data->remote_name, ref, checksum);
         }
     }
 

src/libostree/ostree-repo.h

@@ -510,9 +510,11 @@ gboolean ostree_repo_prune (OstreeRepo        *self,
 /**
  * OstreeRepoPullFlags:
  * @OSTREE_REPO_PULL_FLAGS_NONE: No special options for pull
+ * @OSTREE_REPO_PULL_FLAGS_MIRROR: Write out refs suitable for mirrors
  */
 typedef enum {
-  OSTREE_REPO_PULL_FLAGS_NONE
+  OSTREE_REPO_PULL_FLAGS_NONE,
+  OSTREE_REPO_PULL_FLAGS_MIRROR
 } OstreeRepoPullFlags;
 
 gboolean ostree_repo_pull (OstreeRepo             *self,