gcal-timeline: completed_calendars should be unsigned

In on_calendar_monitor_completed_cb() we have this assert:

g_assert (self->completed_calendars <= g_hash_table_size (self->calendars));

That is comparing a signed integer to unsigned. This is dangerous. We
could add casts to say that we really want to do it, but
completed_calendars should never be negative, so might as well use
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