tests/test-revlog-v2.t

copies: add config option for writing copy metadata to file and/or changset

This introduces a config option that lets you choose to write copy
metadata to the changeset extras instead of to filelog. There's also
an option to write it to both places. I imagine that may possibly be
useful when transitioning an existing repo.

The copy metadata is stored as two fields in extras: one for copies
since p1 and one for copies since p2.

I may need to add more information later in order to make copy tracing
faster. Specifically, I'm thinking out recording which files were
added or removed so that copies._chaincopies() doesn't have to look at
the manifest for that. But that would just be an optimization and that
can be added once we know if it's necessary.

I have also considered saving space by using replacing the destination
file path by an index into the "files" list, but that can also be
changed later (but before the feature is ready to release).

Differential Revision: https://phab.mercurial-scm.org/D6183

#require reporevlogstore

A repo with unknown revlogv2 requirement string cannot be opened

  $ hg init invalidreq
  $ cd invalidreq
  $ echo exp-revlogv2.unknown >> .hg/requires
  $ hg log
  abort: repository requires features unknown to this Mercurial: exp-revlogv2.unknown!
  (see https://mercurial-scm.org/wiki/MissingRequirement for more information)
  [255]
  $ cd ..

Can create and open repo with revlog v2 requirement

  $ cat >> $HGRCPATH << EOF
  > [experimental]
  > revlogv2 = enable-unstable-format-and-corrupt-my-data
  > EOF

  $ hg init empty-repo
  $ cd empty-repo
  $ cat .hg/requires
  dotencode
  exp-revlogv2.1
  fncache
  sparserevlog
  store

  $ hg log

Unknown flags to revlog are rejected

  >>> with open('.hg/store/00changelog.i', 'wb') as fh:
  ...     fh.write(b'\x00\x04\xde\xad') and None

  $ hg log
  abort: unknown flags (0x04) in version 57005 revlog 00changelog.i!
  [255]

  $ cd ..

Writing a simple revlog v2 works

  $ hg init simple
  $ cd simple
  $ touch foo
  $ hg -q commit -A -m initial

  $ hg log
  changeset:   0:96ee1d7354c4
  tag:         tip
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     initial
  
Header written as expected

  $ f --hexdump --bytes 4 .hg/store/00changelog.i
  .hg/store/00changelog.i:
  0000: 00 01 de ad                                     |....|

  $ f --hexdump --bytes 4 .hg/store/data/foo.i
  .hg/store/data/foo.i:
  0000: 00 01 de ad                                     |....|