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-pull.c

@@ -96,6 +96,7 @@ typedef struct {
   guint64       previous_bytes_sec;
   guint64       previous_total_downloaded;
 
+  GError       *cached_async_error;
   GError      **async_error;
   gboolean      caught_error;
 
@@ -1697,7 +1698,10 @@ ostree_repo_pull_with_options (OstreeRepo             *self,
   pull_data->is_mirror = (flags & OSTREE_REPO_PULL_FLAGS_MIRROR) > 0;
   pull_data->is_commit_only = (flags & OSTREE_REPO_PULL_FLAGS_COMMIT_ONLY) > 0;
 
-  pull_data->async_error = error;
+  if (error)
+    pull_data->async_error = &pull_data->cached_async_error;
+  else
+    pull_data->async_error = NULL;
   pull_data->main_context = g_main_context_ref_thread_default ();
   pull_data->flags = flags;
 
@@ -2227,6 +2231,15 @@ ostree_repo_pull_with_options (OstreeRepo             *self,
 
   ret = TRUE;
  out:
+  /* This is pretty ugly - we have two error locations, because we
+   * have a mix of synchronous and async code.  Mixing them gets messy
+   * as we need to avoid overwriting errors.
+   */
+  if (pull_data->cached_async_error && error && !*error)
+    g_propagate_error (error, pull_data->cached_async_error);
+  else
+    g_clear_error (&pull_data->cached_async_error);
+    
   ostree_repo_abort_transaction (pull_data->repo, cancellable, NULL);
   g_main_context_unref (pull_data->main_context);
   if (update_timeout)