fetcher: Hold a ref to main context for lifetime of thread
fetcher: Hold a ref to main context for lifetime of thread

I don't think this fixes the bug I was seeing, but it makes me more
comfortable to know we have a strong ref to the main context across
the thread lifetime, and we only unset the default right before
we go away.

If something in `thread_closure_unref()` used
`g_main_context_get_thread_default()` for example it'd be wrong
before.

Closes: #383
Approved by: mbarnes

gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree