branchcache: use an explicit class for the v2 version
This prepare the introduction of an experimental v3 format version.
In the process, we move the description of the format in that new class.
$ cat <<EOF >> $HGRCPATH
> [extensions]
> hooklib =
>
> [phases]
> publish = False
> EOF
$ hg init a
$ hg --cwd a debugbuilddag .
$ hg --cwd a phase --public 0
$ hg init b
$ cat <<EOF >> b/.hg/hgrc
> [hooks]
> pretxnclose-phase.enforce_draft_commits = \
> python:hgext.hooklib.enforce_draft_commits.hook
> EOF
$ hg --cwd b pull ../a
pulling from ../a
requesting all changes
adding changesets
adding manifests
adding file changes
error: pretxnclose-phase.enforce_draft_commits hook failed: New changeset 1ea73414a91b in phase 'public' rejected
transaction abort!
rollback completed
abort: New changeset 1ea73414a91b in phase 'public' rejected
[255]
$ hg --cwd a phase --force --draft 0
$ hg --cwd b pull ../a
pulling from ../a
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 0 changes to 0 files
new changesets 1ea73414a91b (1 drafts)
(run 'hg update' to get a working copy)
$ hg --cwd a phase --public 0
$ hg --cwd b pull ../a
pulling from ../a
searching for changes
no changes found
error: pretxnclose-phase.enforce_draft_commits hook failed: Phase change from 'draft' to 'public' for 1ea73414a91b rejected
abort: Phase change from 'draft' to 'public' for 1ea73414a91b rejected
[255]