Please don't update package not for buster to unstable

[Shengjing Zhu <zhsj@debian.org>]

Hi Anthony,

I see you uploaded golang-github-mattn-go-isatty/0.0.7-1 and
golang-golang-x-sys/0.0~git20190412.9773273-1 to unstable. I'm sure
release team won't unblock these new versions for buster.

This breaks the build-rdepends, if they need to fix RC bug in buster.
Take aptly for example, I upload a fix for buster, now it can't
migrate because it should migrate after golang-github-mattn-go-isatty
and golang-golang-x-sys.

Maybe we can use testing-pu, but the release team strongly prefer
changes that can be done via unstable instead of
testing-proposed-updates[1].

[1] https://release.debian.org/buster/freeze_policy.html

PS, it's not about packages not in testing.

-- 
Shengjing Zhu