node: introduce nodeconstants class
In preparing for moving from SHA1 hashes to a modern hash function,
place nullid and other constant magic vules in a class. Provide the
active set of constants in the repository and push it down. Provide
nullid directly in strategic places like the repository as it is
accessed very often. This changeset introduces the API change, but not
the mechanical replacement of the node.py attributes itself.
Differential Revision: https://phab.mercurial-scm.org/D9750
== New Features ==
* `hg purge` is now a core command using `--confirm` by default.
* The `rev-branch-cache` is now updated incrementally whenever changesets
are added.
* The new options `experimental.bundlecompthreads` and
`experimental.bundlecompthreads.<engine>` can be used to instruct
the compression engines for bundle operations to use multiple threads
for compression. The default is single threaded operation. Currently
only supported for zstd.
== New Experimental Features ==
* There's a new `diff.merge` config option to show the changes
relative to an automerge for merge changesets. This makes it
easier to detect and review manual changes performed in merge
changesets. It is supported by `hg diff --change`, `hg log -p`
`hg incoming -p`, and `hg outgoing -p` so far.
== Bug Fixes ==
== Backwards Compatibility Changes ==
* In normal repositories, the first parent of a changeset is not null,
unless both parents are null (like the first changeset). Some legacy
repositories violate this condition. The revlog code will now
silentely swap the parents if this condition is tested. This can
change the output of `hg log` when explicitly asking for first or
second parent.
== Internal API Changes ==
* `changelog.branchinfo` is deprecated and will be removed after 5.8.
It is superseded by `changelogrevision.branchinfo`.
* Callbacks for revlog.addgroup and the changelog._nodeduplicatecallback hook
now get a revision number as argument instead of a node.
* revlog.addrevision returns the revision number instead of the node.
* `nodes.nullid` and related constants are being phased out as part of
the deprecation of SHA1. Repository instances and related classes
provide access via `nodeconstants` and in some cases `nullid` attributes.