tests/test-convert-identity.t

mergestate: introduce a new ACTION_KEEP_NEW

`ACTION_KEEP` is overloaded and it's hard to figure out how we end up with this
KEEP, what was the state of things.

In a previous patch, we introduced `ACTION_KEEP_ABSENT` which represents files
which are kept absent in the working directory.

There is another special case where we keep the file when it's not present on
both ancestor and remote side. We introduce a dedicated action for that.

The goal is to use these information to make bid merge smarter.

Differential Revision: https://phab.mercurial-scm.org/D9002

Testing that convert.hg.preserve-hash=true can be used to make hg
convert from hg repo to hg repo preserve hashes, even if the
computation of the files list in commits change slightly between hg
versions.

  $ cat <<'EOF' >> "$HGRCPATH"
  > [extensions]
  > convert =
  > EOF
  $ cat <<'EOF' > changefileslist.py
  > from mercurial import (changelog, extensions, metadata)
  > def wrap(orig, clog, manifest, files, *args, **kwargs):
  >   files = metadata.ChangingFiles(touched=[b"a"])
  >   return orig(clog, manifest, files, *args, **kwargs)
  > def extsetup(ui):
  >   extensions.wrapfunction(changelog.changelog, 'add', wrap)
  > EOF

  $ hg init repo
  $ cd repo
  $ echo a > a; hg commit -qAm a
  $ echo b > a; hg commit -qAm b
  $ hg up -qr 0; echo c > c; hg commit -qAm c
  $ hg merge -qr 1
  $ hg commit -m_ --config extensions.x=../changefileslist.py
  $ hg log -r . -T '{node|short} {files|json}\n'
  c085bbe93d59 ["a"]

Now that we have a commit with a files list that's not what the
current hg version would create, check that convert either fixes it or
keeps it depending on config:

  $ hg convert -q . ../convert
  $ hg --cwd ../convert log -r tip -T '{node|short} {files|json}\n'
  b7c4d4bbacd3 []
  $ rm -rf ../convert

  $ hg convert -q . ../convert --config convert.hg.preserve-hash=true
  $ hg --cwd ../convert log -r tip -T '{node|short} {files|json}\n'
  c085bbe93d59 ["a"]
  $ rm -rf ../convert