[pkg-go] Bug#956299: golang-google-grpc update broke gitaly

[Pirate Praveen]

[follow up on #956299]

Hi Dmitry,

I was preparing gitaly 12.9.2 (gitaly master branch) and was building 
fine till yesterday but started ftbfs from today. See
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=956299 for error 
message.

I cannot upload newer gitaly to unstable till rails 6 migration starts 
(ruby component of gitaly depends on rails) so you did not see the 
build dependency (the version in unstable vendors grpc). I don't know 
if there is an easy way to check if a version in experimental is in 
build depends.

Can you help fix the failure? Also it will be helpful if you can 
consider gitaly in experimental for future updates.

Thanks
Praveen