Branches webrevs

[Aleksey Shipilev]

Hi,

In Shenandoah project, we eventually figured that having the automatically-generated webrevs against
upstream is a very convenient tool for debugging (figuring what exactly was changed), review
(sometimes, accidental changes leak in, and cause head-scratchy bugs), and merges (because you can
tell if the change is coming from your code).

This is the example for amber-demo branch (among other patches for other projects):
  https://builds.shipilev.net/patch-openjdk-amber-amber-demo/

If you tell me what other branches are interesting to have, I can add them to the schedule as well.
The rule of thumb is to get the more or less stable branches that are slated for merge to upstream
at some point.

(Technically, webrevs are generated against the first "hg in" changeset's parent, which assumes the
linear history in "default" branch, and that all branches get merged from default regularly. I think
that is already happening in amber).

-Aleksey