[Development] [HEADS-UP] Updates to branching scheme

[Oswald Buddenhagen]

hello,

as discussed at the QtCS and these lists, forward-merging from the LTS
branch 5.6 is becoming a significant burden.
therefore, 5.6 is switching to a cherry-pick based model:
- 5.6 is *NOT* going to be forward-merged any more, *ever* (also not to
  merge tags)
- you may integrate only changes which have been already integrated into
  a stable mainline
- use cherry-pick -x. don't forget to refresh the change if you create
  it before the source change integrates (new sha1!).
- within the 5.6 "realm", there will be forward-merging from the release
  branch (next: 5.6.3) to 5.6 itself, so cherry-pick to only one of the
  5.6.x branches.
- this model is actually the same as what we employed for 4.8, our last
  LTS branch.
- if you find that your changes recently integrated on 5.6 don't show up
  on 5.7 until monday, it means that you missed the last merge. in this
  case you'll need to _forward_-cherry-pick them. this is an exceptional
  case. to prevent this from happening from this point on, staging in
  5.6 is temporarily locked down until we open it up again for the
  cherry-picks.

second item: with the 5.8.0 release approaching, 5.7 is now reaching end
of life. only critical fixes should go to 5.7 at this point. staging in
5.7 has been restricted to the release team, so we will be able to
forward-merge 5.7 directly into 5.8.0, and we could in principle do a
5.7.2 release directly from that branch. all non-critical fixes should
go to 5.8 now.

i'm expecting a flurry of retargeting requests of changes from 5.6 and
5.7 to 5.8 now.

regards,
lars & ossi