bts-link-upstream treats Closed on GH as fixed-upstream

[Adrian Bunk]

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=828556;msg=16

Upstream closed the upstream issue on GitHub with a message
that could be summarized as "this will never be fixed".

"Merged" can be treated as "fixed-upstream", but for "Closed"
it cannot be assumed that the issue was actually fixed and not
rejected.

Thanks
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed