pull: Go back to using one main context

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)