CONTRIBUTING

sidedata-exchange: rewrite sidedata on-the-fly whenever possible

When a A exchanges with B, the difference of their supported sidedata categories
is made, and the responsibility is always with the client to generated it:
    - If A pushes to B and B requires category `foo` that A does not have, A
      will need to generate it when sending it to B.
    - If A pulls from B and A needs category `foo`, it will generate `foo`
      before the end of the transaction.
    - Any category that is not required is removed. If peers are not compatible,
      abort.

It is forbidden to rewrite sidedata for a rev that already has sidedata, since
that would introduce unreachable (garbage) data in the data file, something
we're not prepared for yet.

Differential Revision: https://phab.mercurial-scm.org/D10032

Our full contribution guidelines are in our wiki, please see:

https://www.mercurial-scm.org/wiki/ContributingChanges

If you just want a checklist to follow, you can go straight to

https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist

If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).