repo: Allocate a tmpdir for each OstreeFetcher to isolate concurrent downloads

This way two pulls will not use the same tmpdir and accidentally
overwrite each other. However, consecutive OstreeFetchers will reuse
the tmpdirs, so that we can properly resume downloading large objects.

https://bugzilla.gnome.org/show_bug.cgi?id=757611

src/libostree/ostree-repo.c

@@ -419,7 +419,9 @@ _ostree_repo_remote_new_fetcher (OstreeRepo  *self,
   if (tls_permissive)
     fetcher_flags |= OSTREE_FETCHER_FLAGS_TLS_PERMISSIVE;
 
-  fetcher = _ostree_fetcher_new (self->tmp_dir_fd, fetcher_flags);
+  fetcher = _ostree_fetcher_new (self->tmp_dir_fd, fetcher_flags, NULL, error);
+  if (fetcher == NULL)
+    goto out;
 
   {
     g_autofree char *tls_client_cert_path = NULL;