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-fetcher.c

@@ -731,6 +731,10 @@ fetch_uri_sync_on_complete (GObject        *object,
   data->done = TRUE;
 }
 
+/* Synchronously request a URI - will iterate the thread-default main
+ * context for historical reasons.  If you don't want that, push a
+ * temporary one.
+ */
 gboolean
 _ostree_fetcher_request_uri_to_membuf (OstreeFetcher  *fetcher,
                                        SoupURI        *uri,
@@ -754,8 +758,7 @@ _ostree_fetcher_request_uri_to_membuf (OstreeFetcher  *fetcher,
   if (g_cancellable_set_error_if_cancelled (cancellable, error))
     return FALSE;
 
-  mainctx = g_main_context_new ();
-  g_main_context_push_thread_default (mainctx);
+  mainctx = g_main_context_ref_thread_default ();
 
   data.done = FALSE;
   data.error = error;
@@ -800,8 +803,6 @@ _ostree_fetcher_request_uri_to_membuf (OstreeFetcher  *fetcher,
   ret = TRUE;
   *out_contents = g_memory_output_stream_steal_as_bytes (buf);
  out:
-  if (mainctx)
-    g_main_context_pop_thread_default (mainctx);
   g_clear_object (&(data.result_stream));
   return ret;
 }