[BuildStream] HACKING.rst policy updates

[Tristan Van Berkom <tristan vanberkom codethink co uk>]

Hi all,

I've made a couple of minor changes to the policies around patch
submission in the HACKING.rst file, here they are:


  Commit message policy
  ~~~~~~~~~~~~~~~~~~~~~
  It is no longer a hard requirement to mention the issue number
  in commit messages, although preferable to do so where appropriate.

  We've enabled the merge commit messages on gitlab, so as long as
  the merge request itself has been set to resolve the particular
  related issues, we can trace the commits back to the issues.

  The motivation for this change is to reduce friction in reviews.


  Rewording of the WIP: merge requests
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  The wording here was a bit too strong I think so I changed this.

  The intention is to allow upstream to efficiently ignore merge
  requests which have been created prior to being proposed upstream.

  Normally this distinction is not needed, but with gitlab people seem
  to like to "work within a merge request", so we still need a way to
  distinguish:

    - A patch submitted for review upstream
    - A branch that someone happens to be working on

  We could alternatively just say "If you're not proposing this to be
  merged, just dont create a merge request", but I think it causes less
  friction overall to just go with what seems to be gitlab's proposed
  workflow.


Cheers,
    -Tristan