pull: cache summary and summary.sig

It allows an optimization to skip the download of the summary file
if its .sig file is unchanged.

Downloading the .sig file is much cheaper than downloading the summary
file from repositories with many branches.

https://bugzilla.gnome.org/show_bug.cgi?id=762973

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

src/libostree/ostree-repo.c

@@ -1795,6 +1795,17 @@ repo_remote_fetch_summary (OstreeRepo    *self,
         goto out;
     }
 
+  if (*out_summary && *out_signatures)
+    {
+      if (!_ostree_repo_cache_summary (self,
+                                       name,
+                                       *out_summary,
+                                       *out_signatures,
+                                       cancellable,
+                                       error))
+        goto out;
+    }
+
   ret = TRUE;
 
  out: