repo: Allow using options for fetching summary

This adds a _with_options variant of the
ostree_repo_remote_fetch_summary function, so we can tell the fetcher
to use a specific URL instead taking it from the remote config.

Closes: #290
Approved by: cgwalters

src/libostree/libostree.sym

@@ -337,3 +337,9 @@ global:
  * Versions above here are released.  Only add symbols below this line.
  *                         NOTE NOTE NOTE
  */
+
+LIBOSTREE_2016.6 {
+global:
+        ostree_repo_remote_fetch_summary_with_options;
+
+} LIBOSTREE_2016.5;