CONTRIBUTING

narrow: the first version of narrow_widen wireprotocol command

This patch introduces a wireprotocol command narrow_widen() which will be used
to widen a narrow copy using `hg tracked` command provided by narrow extension.

The wireprotocol command takes the old and new includes and excludes, common
heads, changegroup version, known revs, and a boolean ellipses and generates a
bundle2 of the required data and send it. The clients receives the bundle2
and applies that.

A bundle2 instead of changegroup because in future we might want to add more
things to send while widening. Thanks for martinvonz for the suggestion.

I am not sure whether we need changegroup version as an argument to the command
as I *think* narrow needs changegroup3 already.

The tests shows that we don't exchange phase data now while widening which is
nice. Also we don't check for pushkeys, rbc-cache, bookmarks etc.

This does not support ellipses cases for now but will be supported in future
patches. Since we send bundle2, it won't be hard to plug the ellipses logic in
here.

The existing code for widening a non-ellipses case is also dropped in this
patch.

Differential Revision: https://phab.mercurial-scm.org/D4813

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).