The build system enforces a limit on the compressed tree tarball size
that it's willing to fetch and propagate throughout the rest of the
system. If that limit is exceeded, consider it a terminal error and
fail the build, since fetching the exact same commit again will not
produce a different result.

Such situations will always involve some human intervention:

• if the limit was exceeded unexpectedly, for example due to unwanted
  content being accidentally checked in, a different commit will be
  pushed and tested

• if enough time has passed that what used to be deemed as enough
  no longer holds, the size limit will need to be updated

Factor in buildgo.FetchSubrepo into the two places in cmd/coordinator.
Its behavior was so trivial (just 2 function calls) that the overhead
of creating an additional abstraction overshadowed any small benefits.

Fixes golang/go#49707.
Updates golang/go#46379.
Updates golang/go#11224.

Change-Id: I7219da97a888ce621d71a51f828400a11e343bf9
Reviewed-on: https://go-review.googlesource.com/c/build/+/365974
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

The x/website repository is unusually large due to having content
from x/blog, and as of CL 365135 also from x/talks. Increase its
limit to 200 MB, but leave other repositories as they were.

Updates golang/go#46379.

Change-Id: Ib18766cb5903e602899c1777f1fba7d7cb2a5767
Reviewed-on: https://go-review.googlesource.com/c/build/+/366135
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Go Bot <gobot@golang.org>