There were a few places in releasebot that assumed the version
being made was a minor release (like go1.2.3) and didn't work
as expected for major releases (like go1.2). This change fixes
those, making releasebot suitable for creating major releases.

Computing the release branch and some of the error checking can
be factored out of doRelease to happen earlier. It's out of scope
for this CL, as it's better suited for a larger refactor later.

Fixes golang/go#34045

Change-Id: Id2204d372043e1fd2afc699d39412677c603f034
Reviewed-on: https://go-review.googlesource.com/c/build/+/193018
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>

Releasebot has achieved the milestone of supporting all release
types some time ago. Let it have this badge of honor, so it can
celebrate and show off to its robot friends.

Also adjust the phrasing in nextStepsPrepare and nextStepsBeta
to be more clear when only the prepare stage has completed.

For golang/go#34045.

Change-Id: Ie4771ab6b6692a26c3363a3a35f131edd2d569cf
Reviewed-on: https://go-review.googlesource.com/c/build/+/239038
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>

The check was added for golang/go#34045, back when doc/contrib.html
lived in the main repo and needed to be manually updated before each
major Go release. This is no longer applicable after CL 229483 and
CL 229485, so remove it before the upcoming Go 1.15 release.

This is compatible with future major releases and does not affect
upcoming minor releases of previous major releases.

Fixes golang/go#40476.
Updates golang/go#34045.

Change-Id: Ifc54dce106633e3d6b9ccdb0db4dae2b7434ce93
Reviewed-on: https://go-review.googlesource.com/c/build/+/245477
Reviewed-by: Carlos Amedee <carlos@golang.org>