pull: Don't execute static deltas when mirroring

We don't presently support this, since the static delta code assumes
it can just `mmap()` file objects.  We could at some point implement
this, but for now just skip executing deltas when doing
`archive -> archive` mirroring.

I noticed this when trying to mirror a repo in Jenkins in
[CAHC](https://wiki.centos.org/SpecialInterestGroup/Atomic/Devel).

Closes: #408
Approved by: giuseppe

src/libostree/ostree-repo-pull.c

@@ -2705,7 +2705,8 @@ ostree_repo_pull_with_options (OstreeRepo             *self,
                                     &from_revision, error))
         goto out;
 
-      if (!disable_static_deltas && (from_revision == NULL || g_strcmp0 (from_revision, to_revision) != 0))
+      if (!disable_static_deltas && !pull_data->is_mirror &&
+          (from_revision == NULL || g_strcmp0 (from_revision, to_revision) != 0))
         {
           if (!request_static_delta_superblock_sync (pull_data, from_revision, to_revision,
                                                      &delta_superblock, cancellable, error))