fetcher: Fix another finalization deadlock

fetcher: Fix another finalization deadlock

If the current repo is already up to date (we have no content to
fetch), it's possible for the fetcher to not request any URIs.  So
create and then finalize it quickly.

Finalization involves calling `g_main_loop_quit()` +
`g_thread_wait()`.  However, if `g_main_loop_quit()` is run *before*
`g_main_loop_run()`, we'll deadlock because GMainLoop assumes in
`_run()` to start things.

This is a common trap - ideally, GMainLoop would record if `_quit()`
was called before `_run()` or something, but doing that now would
likely break people who are expecting quit() -> run() to restart.

In general, we've moved in various GLib-consuming apps to an
explicit "main context iteration with termination condition" model;
see `pull_termination_condition()` in the pull code.

This fixes this race condition.

I verified that an assertion in `_finalize` that more than
zero URIs were requested was hit in multiple test cases, and this patch
has survived a while of make check loops.

Closes: https://github.com/ostreedev/ostree/issues/496
Closes: #499
Approved by: jlebon