cmd/go: tighten the check for pseudo-version base tags
Do not allow a pseudo-version derived from a canonical tag to refer to
the same revision as the tag itself. It's unnecessary (because
canonical tags already have a total ordering) and confusing (the
pseudo-version appears to come after the tag, but actually refers to
the exact same revision).
Updates #32879
Updates #27173
Change-Id: I02befedbe89c8819bdd93e470783ce63fc813193
Reviewed-on: https://go-review.googlesource.com/c/go/+/184720
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Jay Conrod <jayconrod@google.com>