pull: Go back to using one main context

xdg-app was hanging for me with v2015.8, but worked with v2015.7.
I narrowed things down to the GMainLoop/context commit, in which
we started pushing a temporary main context for synchronous
requests internally.

That's never really going to work with libsoup - there needs
to be a single main context which works on the socket.  Furthermore,
clients couldn't get progress messages that way.

For *other* internal uses where we added APIs that talk to the remote
repo, we cleanly push a temporary main context.

(Note that I kind of snuck in a change here around the GError handling
 in pulls that isn't strictly related but came up in testing)

src/libostree/ostree-repo.c

@@ -1681,6 +1681,7 @@ _ostree_preload_metadata_file (OstreeRepo    *self,
                                GError        **error)
 {
   gboolean ret = FALSE;
+
   if (is_metalink)
     {
       glnx_unref_object OstreeMetalink *metalink = NULL;
@@ -1740,12 +1741,16 @@ repo_remote_fetch_summary (OstreeRepo    *self,
                            GError       **error)
 {
   glnx_unref_object OstreeFetcher *fetcher = NULL;
+  g_autoptr(GMainContext) mainctx = NULL;
   gboolean ret = FALSE;
   SoupURI *base_uri = NULL;
   uint i;
   const char *filenames[] = {"summary", "summary.sig"};
   GBytes **outputs[] = {out_summary, out_signatures};
 
+  mainctx = g_main_context_new ();
+  g_main_context_push_thread_default (mainctx);
+
   fetcher = _ostree_repo_remote_new_fetcher (self, name, error);
   if (fetcher == NULL)
     goto out;
@@ -1787,9 +1792,10 @@ repo_remote_fetch_summary (OstreeRepo    *self,
   ret = TRUE;
 
  out:
+  if (mainctx)
+    g_main_context_pop_thread_default (mainctx);
   if (base_uri != NULL)
     soup_uri_free (base_uri);
-
   return ret;
 }